Seeing how SEGA is crackdowning on some images "cause they degrade the public's opinion of the game" (not an exact quote) I thought this question is worth answering...
I know there is not only one factor that makes one buy a game, but which one is the One for you, the main attraction, the thing that makes you wait and be interested in our beloved MTW2...
1) The previous installments in the TW series - acronyms like STW, MTW, MTW:VI, RTW and RTW:BI have no secrets for you. As you loved each and all of them you shall love MTW2...
2) The graphics - looking at flashy units doing killer animations turns you on. And as strategy begins to greatly resemble FPS you are strangely drawn to MTW2...
3) The historicity of it all - you know your Alexanders, Hannibals, Romans and Arveni and wanna get to know the Joan d'Arc's, Venetians and Lionheart's MTW2 shall put on offer...
4) The modability of the game - you loved BKB work, RTR and/or EB, Darthmod and SPQR and now there's a new playground out there called MTW2...
5) Raving reviews - you only buy what the little voices tell you too...
6) Nice pre-release pictures and a good demo - you like to try things on for size before making a purchase...
7) Other reasons - oh, please, do be specific...
PS: If anyone is wondering I am a long time user and abuser of this game. When I am 50 you will probably find me playing Space Age: Total War and telling my nephews how I took over Europe with, well, everyone...
But I am in it for the historical feel and also for the brilliant stuff modders can produce...