Lets see what happened in Kosovo: it was a traditional Serbian territory, but in the last few decades it got mostly populated by Albanians. When they wanted to separate, the Serbs didn't like it, hence the war. NATO came and facilitated the separation.
What are the similarities with Ukraine: Crimea is traditional Russian territory and almost exclusively populated by Russians; the rest of Eastern Ukraine has also a very sizable Russian population and is pro-Russian - they want to separate and join Russia, which is also in Russia's interest. The rest of Ukraine doesn't like it, hence the current conflict.
The way I see it is, if USA and NATO want to apply the same criteria, they should allow Eastern Ukraine to separate and join Russia, if that's what the local population wants.
That's what Serbs say, but it isn't the case. From registration more than a hundred years ago:
Austrian registration (1899):
Albanian 47.88%
Serbian 43.7%
British estimation (1906)
Albanian 2/3
Serbian 1/3
Noel Malcolm estimation for 1912 (based on a few Serbian stats)
Albanian around 70%
Serbian 25%
1921:
Albanian 65.8%
Serbian 26%
1931 (Kingdom of Yugoslavia registration):
Albanian 60%
Serbian (and other slavic population) 32.64%
1948 estimation:
Albanian 68.46%
Serbs 23.62%
What really happened, is that after the constitution of 1974, Albanian rights increased on Kosovo. Albanians could run and become mayors, ministers etc. The University was open on the Albanian language. While local Serbs had slightly more rights, they weren't exactly content with this and a lot of them left Kosovo (cause they didn't want to be ruled from Albanians). On the other side, Albanians natality is much higher than that of Serbians which made stats change (on 1991 there are estimated to be around 85% Albanian and around 10% Serbs).
When Milosevic came to power he went batshit crazy and one of the first things he did was to change the constitution, revoking the rights for Albanians. University was closed as were closed high schools (to his credit, he allowed primary schools continue being open), which made Albanian start parallel education. A few hundred thousand people lost their job cause they were Albanian, and there started a big presecution of Albanian population. A lot were beated, some of them to death for no reasons at all. After 7 years of peaceful resistance (lead by president of Kosovo whosomehow weirdly thought that he can copy Ghandi and be succesful on it), the first armed resistance began. Serbians responded killing quite a lot of Albanians, making a few massacres where entire villages were executed and the leader of mission of OSCE called that in one of them had happened 'crime against humanity'.
Then in Rambouille happened a meeting between Albanian leaders (both from KLA and from president which generally were against each other) and Serbian leaders. Serbia didn't accept to change anything and they didn't sign the proposed peace treaty. On the other hand, they intensified killing of people. Kosovan Liberty Army also finally managed to be armed much better, though still not a match for Yugoslavian army. And so, NATO (pretty much US, UK and Turkey) had to intervene otherwise it would have happened even worse than in Bosna. When the war ended, around a million Albanians (50% of the entire population) had already left Kosovo, and around 12000-15000 were killed. 2000 of bodies haven't still be found.
Then after a few years started a process of negotiations between Serbia, Albanian leaders in Wiena for the final status of Kosovo. It was lead by Marti Ahtisari (ex prime minister of Finland). After 2 years of negotiations he proposed indipendence for Kosovo, while Serbian population will have a lot more rights than minorities around the world (positive discrimination). Serbia didn't accept that, Russia threatened for veto, which means that Kosovo had to declare indipendence without going to Security Council. So far had been recognized from 110 states, including but not limited to US, Turkey, Australia, Canada, Japan, South Korea and 23 out of 28 EU coutries (all bar Spain, Slovakia, Greece, Cyprus and Romania).
To relate the situtation - why I don't care at all what happend on Crimea - the situations are completely different. Ukraininan state and army hasn't been killing Russians in Crimea and east Ukraine. They weren't persecuted them. They didn't fire Russian population from their jobs for being Russians. Despite what Vladimir Putin said about similarities between Kosovo and Crimea (he said the same in war of Georgia for similarities between Kosovo and Abkhazia/Osetia), there aren't no similarities at all. Also, considering how devoted is Putin when it comes to sovereinity of states (he threatened that Russian will veto Kosovo if they want to enroll on UN), it is quite weird seeing him invading countries, eventhough the situation there was more softer than it was in Kosovo.
While I don't have nothing against East Ukraine joining Russia (I believe on nations' right for indipendence), the problem is that there are a lot of similar situations all over the world, and in some way it will open Pandora box. So bar wars or humanitary chatastrophes (which happened both in Kosovo and Bosna), I am not sure that it is a right idea to redraw the borders of Europe. Obviously, Putin doesn't give a shit about Russian population there, and this is more a territorial war and also a showing of muscles.