My theory: Putin is bluffing, and he's doing it very well, but he might be wrong.
Think about it. 40,000 troops on the border. War chanting at home. Mentioning Transnistria. The whole occupying Crimea thing. The west (and everyone else) is convinced he has his eyes on Ukraine, Poland, the Baltic states, something.
But what if his plan is to act out as if he was going to go around and murder everyone, but instead seemingly "settle" for Crimea alone? Here, he gets to deescalate the situation, and on his own terms, without letting everyone think he is a ruthless dictator plotting to conquer everyone. The west's main objective, maintain the rest of Ukraine, is achieved. No reason to go to war or anything; he must know that it would be suicidal in the long run to fully engage NATO. So instead, he pulls back, the west returns to a general state of disquiet but not alarm, and he manages to make off with Crimea while managing to look like he's being conciliatory and making the EU look weak. We already see some of this in Putin's surprise call to Obama. He literally gets away with Crimea while conceding, which is impressive. No one wants to help Ukraine get that back if Putin isn't going to go nuts and invade the rest.
He might've miscalculated though. There is a lot of antipathy towards him now here; the next President, whether Democrat or Republican, isn't going to be trying any sort of "reset". The russo-neighbors are scared, and many will accelerate their pull away: fear of invasion has a tendency to silence those partial to the potential invaders. Whether or not he plans on fully conceding the rest of his influence in Ukraine, there is a lot of anger towards him there. And no one remembers when, just a short while ago, he was the champion of freedom from spying, and Snowden is worse off for being associated. NATO is re-ascendant in it's old role, where just weeks before it was disengaging. Only time will tell whether this will develop into the new norm though.
I might be wrong mind. He could be the wild nationalist tiger I've seen him depicted as. In that case, may god help us all. But I don't buy it.