The concern is about whether Putin will want to stop at annexing Novorussiya and confirming Crimea. He may, for example, want to secure a land connection to Crimea, which means taking all or most of the eastern bank of the Dnieper. Of course, the more "core" Ukrainian land he takes, the more he risks a long-term insurgency against Ukrainian partisans, a war that that is likely to be highly unpopular in Russia and one that will damage the stability of the Federation, especially if it inspires the Chechens and other dissident groups to give uprisings another shot. Furthermore, and particularly if Ukraine completely folds within the first days of the attack, it is likely he wish push on to Kiev and establish a puppet regime against the sovereign wishes of the rest of Ukraine.
I don't have high hopes for Ukraine in this regard; the Russians have an overwhelming superiority of armor and aircraft, both of which will excel on the Ukrainian plains. Russian air superiority will be assured, and as we've known since World War II, whoever holds air superiority also controls the battle.