Now, each faction in vanilla has its own roster, and in many ways its own feel, combined with the campaign position. I'll try to surmise them here, so that you can comment and point any mistakes i've made in them.
seleucidare favorite for many people in PvP - great roster with catapharcs, legions, phalanxes and elephants. But they are expensive, in the campaign they have it quite hard, not to mention, that their territory is HUGE. Good luck getting those reinforcements from your capitol to the war with parthia... Not to mention, that if i say that the selucid have bad infrastructure, i'll be offending the bad infrastructure as a whole! They have no MIC's above militia barracks, almost no population, no roads, no armies and no money! But oh, boy do they have enemies! I still haven't seen them as a real power in my vanilla games.
Macedons have good phalanx units, and quite the good cavalry, but lack ranged and special units. Their roster isn't that bad, but they start surrounded and with the brutii at the back door. Now, having all three phalangite units may sound nice and all, adding to the fact that you have great cavalry options (well... exept greek cavalry - they suck) but you get the brutii a few turns after the start. Have fun surviving. Yes, sometimes, they manage to spread out, to capture some regions... but even then, the romans crush them as bugs.
GCS are screwed big time. They don't have pike man, do have decent infantry, but man do they lack anything else - bad cavalry, bad ranged units, and no pikes, so that the phalangites can crush them for fun. That said, they do manage to survive long time. guess it's because the AI doesn't attack islands well. In a way the hoplites are one of the best units in the game - they can form a phalanx against normal infantry, and they can fight in non-phalanx order against the phalangites... but they do lose to the elites.
Pontus has one of the few really interesting rosters in the game. I actually like it a lot. Hillmen, may be week, but can screen your phalangites quite well. And really good heavy cavalry. Going to there is quite hard, tho. You start with two useless regions, and nothing really big to conquer around you. Yeah, remember those eastern infantry units, that die from almost anything besides peasants? Yeah, that will be your main infantry unit for a long time. Combined with javelin cavalry... If they survive, tho, pontus some times becomes a strong faction, conquering all of Anatolia, and some times even beating Armenia (what's with those two? Why do they rarely beat each other?) and holding egypt at bay.
Armenia is similar to Pontus. And similar to Parthia. Mainly getting the bad sides of each. You do start with the painful to watch eastern infantry and hillmen, and in 3rd tire you get a spear unit. Well it is practically an Eastern Hoplite Analog (both in use and in stats), but you have something, that does not die if it trips, and does not run the moment someone sneezes. That said, it has short spear. Properly used pikes slaughter him. And after a while you'll get... Armenian Legionaries. Now, i have no idea who got that idea and from where. But as a 4th tire unit, you get a nerfed version of the 3rd tire Early legionary Cohort. Armenia has good cavalry... if you consider cavalry archers, followed by cataphract archers and then by real cataphracts - good. I personally don't get the cataphract archers.
Parthia. Now, that is a slow game. Forget about assaulting towns. Everything can brake your main infantry units. Well... except the early infantry of Pontus and Armenia, so you can feel good about that... while watching how the phalangites slaughter your hillmen. Oh, i'm not fair, i know. Counting on the impressively weakened state of the Selucid, the Pathians can deal some major damage and take a lot of regions. And yes - with a siege assault. Mostly, 'couse you can throw away those easterns with ease, and even retrain them everywhere! The waiting to get the cataphracts is a long one tho. Still, best heavy cavalry in the game is yours. Deal with the fact, that even the Scythian axemen are better than your infantry.
Now, all of the above have a peltast and a generic archer unit. Some even have slingers. Nothing special and honestly quite boring.
Egypt is a power. Now, slide aside the main problems with it - that it looks a millennium older than it should, it does somewhat play as a hellenic faction - you have your militia, regular and elite spearmen-infantry (Nubian Infantry, Nile Spearmen and Pharaoh's Guard), supported with irregular troops - desert axemen, bowmen, slingers and so on. They do have an elite archer unit, witch is something different from the others, and they do have those crazy units as camel archers, chariots and so on, but still... oh, yeah, and they are loaded with money! And Kick some major ass, even with the romans around.
Note: Well, they will be overhauled, but the vanilla egypt is quite the interesting to play faction.
Romans are bad-asses that kick the but of any infantry in theory (and in practice, after the reforms), but cost a lot, have no real good counter to cavalry of any type and their garrisons are week big time. Well, the AI is dumb, but a player could find a way to exploit that and smack them around. The truth is, that they could die easy up until the mid-game, if the re weren't 4 of them and the hastati weren't second tire unit. The reforms give them some of the best units in the game. The idea probably was, that the civil war should weaken them, and the other factions can get stronger then, but it does not work. Even when the dumb AI controls them, the romans are a pain. After the reforms, they have one of the best archers, peltasts and cavalry units in the game. Just think about that. And remember - this is from around 18 factions (not counting the senate and the other two houses).
Carthage looks somewhat good, but still has it's problems. Manly with infrastructure as all civilized factions. they have three tires of infantry, that i don't call useless, only because there is always the eastern infantry to hold that title. Iberian infantry are good light infantry, but they are no match for the hastati or any phalanx-capable unit. The Lybian Spearmen guys aren't that much better. Then you finally get something that's worth it - Poeni infantry and sacred band. Now, the sacred band is one of the best units in the game.but chances are, when you need it (and that is when you fight Rome) - you won't have it. Decent cavalry for a western faction, and elephants! Lots and lots of elephants!!! Use them wisely and you can compensate for the lack of good infantry. Use them not so wisely and you'll see what can an elephant do to weak infantry... in your own lines!
PS; No archers. You know the secret of the bow (you have archers on the elephants) but you don't have archers. Don't ask me why!
Numidia. Now if you are searching for a week faction - you've found it! You don't start with infantry, you start with skirmishers! You do however have some decent infantry (desert infantry). But to get really good units you need 4th level settlement. Guess what? Yeah, you probably wont get one, before the romans come and kick your ass. Decent cavalry, elephants and early archers. Not a bad roster, just not a good one, when romans are close early in the game.
Thrace is a really strange faction. Good pikes, combined with falxmen and other good attack units. They compensate with infantry in place of the cavalry needed for the hammer and anvil tactics.And they don't really have anything else - infantry, more infantry and some bad cavalry and bad ranged part. They die almost always somewhere in the early game. Playing as them is fun, but the AI is horrible at controlling them.
Now we star the glass-cannon section of rtw. All barbarian rosters are like that - high attack and low defense, witch gives you not much to work with. And makes one of the best tactics with them: harass, close in, warcray, charge. You'll have the superior numbers, so flank them as hell. And still, different factions have different style.
Gaul. You have one of the best archer units in the game. harass as hell, killing as many as possible and then just charge. Great roster, with druids for balance of the bad moral most of your units have. You only need 3rd level settlement to get the chosen swordsmen and noble cavalry, and then you can go and crush everything around you. You start with good infrastructure for a barbarian faction, and quite a lot of rebel-space to expand in to. You can recruit naked fanatics, but don't rely on them much - they kill a lot, and die a lot. The gauls are balanced, standing up in archery, having good infantry and cavalry and missing only one thing - good spear unit.
Germans - or how to play as a hellenic faction as a barbarian. No, just kidding. But you can use the hammer and anvil tactics quite often. Actually with you starting with a phalanx unit that is a valid tactic a lot of time, until you get higher level settlements. After that, you have the best shock infantry in the whole game - berserkers! Supported with axemen and chosen axemen, you can do a ton of damage. And to add to that - you have a crazy-good heavy cavalry unit, and chosen archers. Only the goddamed infrastructure stops you from using all that to rule the world. Well, that and the fact, that you don't have a lot of armor. You have none, actually. You do have the somewhat-comic unit - screeching women, that can rise your morale and lower the enemies. Oh, and they are a reall glass-cannon. Charge with them at the right time, and they do kill a lot. let anything attack them, and they die as peasants.
Dacia - now, this faction does not look that much at the start, right? Well, guess again, that is one of the few factions, that even the dumb AI has difficulty screwing up! They have a warband unit - normal thing. And they have, right after it a Falx unit! Now, if you haven't seen what a falx unit does to anything, that does not hold a spear to it's face - go try it. And chosen swordsmen right after that. Oh, i forgot. You start with archers and then get chosen archers! What that means is, that you have a good counter of the fact, that your heavy attack units die from archers, slingers and the like easily - you just kill them with your archers. Oh, yeah, and they do have the noble barbarian cavalry, that kicks ass. So, forget about routing the archers with your own cavalry.
Briton. Well, this is a faction a refuse to play. Tehy are just like gauls in the infantry - swordsmen (regular and chosen) and warband. Naked fanatics are replaced with the woad warriors that die from... well, anything. 4 defense. The barbarian PEASANTS have as much! Don't use them around thorny bushes - there is a chance they'll hurt themselves and die. Oh, and you have slingers and the dumbest unit in the game - head hurlers. They throw AP heads on your enemies. With 17 ranged attack. Urban cohorts have 18 ranged attack. Praetorians have 16. Also - you have no cavalry, but you DO have chariots. And simillar to carthage - you keep your archers for them and don't field them separately.
Spain. Now, Spain is really a different faction from the other barbarian ones. Your infantry roster starts with two units, that can't hold a candle to the other barbarian ones - the carthaginian town militia and Iberian infantry. And then (if you are alive at that time) you get the goodies! Scutarii are basically principi with more attack, and less defense. They have the AP pilla as a ranged weapon and kick major ass. And THEN you have the Bull Warriors. And the rest is history. Well, not exactly. You have the round and longshield cavalry, but they aren't very good. And you too don't have archers. At least none of your units shoots a bow, so that we can assume, that Iberians, just can't make bows and arrows.
Scythia. Now here is a crazy faction. You have only one infantry unit, and that's axemen. And then you have archers and chosen archers. And then there is the cavalry... You basically have all the types of cavalry, that you may want - you have missile cavalry (a lot of it), light cavalry, medium AP cavalry and heavy lance cavalry. You'll find out quickly, that two of those units are woman units. Interestingly enough the head hunting maidens are a good unit, that can cut trough armor with ease, and the noble women may have weaker stats than the noblemen, but are fast as hell.