For those who don't understand the larger-scale reasons for why this whole thing may be bad:
1) It destroys the whole nuclear non-proliferation thing. Remember that Ukraine gave up it's nukes in exchange for a treaty that said Russia would NOT do what they are rather explicitly doing. The thing they would not be doing if Ukraine had not given up it's nukes (probably). All those non-nuclear states are suddenly going to see the idea of obtaining their own nukes as a LOT more attractive, and if they've still got any they are gonna hold them as tightly as possible. The idea of giving up your nukes in exchange for assurances is now completely, 100% dead.
2) It establishes that we are perfectly okay with Russia carving off chunks off one of their neighbours. I'm not sure how the historical memory is for some of those here who are saying it's not so bad (or worse than the actions of other countries in recent years), but this is literally how World War II got started, with what amounted to an official policy of appeasement, in Manchuria and the Rhineland. If Russia encounters no existence here, such activity will be considered acceptable - the Overton Window will shift. This is... bad. Exceptionally bad. Escalation is the major worry here - with as tepid a response as we have seen, why would any rational expansionist leader NOT push further? If Russia stops with Crimea, maybe this worry will be allayed.. but with the minimal amount of opposition they are getting here, why would they stop? The US may have gotten cocky, but thanks to their complete incompetence handling Iraq and Afghanistan, both conflicts destroyed our desire for intervention overseas as a nation for at least a little while - Libya was only palatable because we didn't send in any troops, and it was still unpopular. Russia seems to be suffering no such weariness, and in fact this action seems only to be feeding into the new Russian mythos. Especially if they manage a completely bloodless annexation, popular support for such activity seems like it will remain quite strong.
The main hope right now is that they secure Crimea, get the port, and that's it. If they make no greater moves, if they are satisfied with that, we're probably okay. I'm just not all that sure that will be the case right now. And if they do decide to invade the Ukraine as a whole, they get into a quagmire situation like the US did in Iraq - but that would be TERRIBLE for Ukraine, unbelievably so.