Russia doing even more to f**k with your plans re: Iran, Syria, Israel, etc. You know, all those really important foreign policy goals that you'd be jeopardizing so Ukraine could join the EU.
As weak as they are, they aren't Iraq or Afghanistan. You can expect a higher level of military opposition. It won't be nearly as easy as rolling over some two-bit dictatorship. You would win, don't get me wrong, but it'll cost you.
Europe can expect to pay out the ass if it wants that natural gas to keep on flowing.
Crimea will turn into a nightmare for Ukraine. You can expect huge instability and armed insurgency for years to come. This spilling over into other regions is absolutely guaranteed, especially if Ukraine takes heavy-handed action against it.
Russia may feel threatened enough to stop acting rationally.
All for having the exact same strategic reality we will have even if we do nothing.