Well, just a little bit of speculation, if we assume Crimea joins the Russian Federation and then it's over and it doesn't escalate in any other way:
Russia would then have isolated itself diplomatically, it has lost any sort of trust from the West and from it's neighbours (especially Ukraine, but possibly even some current allies), and that will last for some time. Ukraine and other countries will seek closer relations with West, and try to join NATO, maybe the EU.
Europe is already looking for other sources of gas and oil, Russia will definitely lose out on that business long-term wise. There may be sanctions, and some of them may stay in place for quite a while.
In the worst case we have another Cold War. NATO troops stationed at Russia's borders, rocket defense systems in Europe.
Also increased distrust will lead to Russia and the West cooperating even less, which will make diplomatic solutions to a lot of crisis or tension situations way more difficult (like Syria and Iran).
Additionally, as has been pointed out before, nuclear non-proliferation has become far more unlikely. (Though I personally didn't believe in it anyway.)
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Since we talked about how there will be no international observers at the referendum, here's a bizarre piece of information (
link in German):
Moscow has invited some European right-wing populist parties as observers, Front National, FPÖ, Lega Nord and Vlaams Belang. FN and FPÖ dislike the EU, the other two are separatists, that's why they support Russia in this. Clearly an attempt to get at least some international recognition, probably not a successful one.