Firstly, warm water ports don't matter anymore. Modern ships can sail through (or, more importantly, under) ice without too much trouble. Secondly, Russia has a warm water port! They already own Sevastapol! Why would they invade Ukraine to gain control of a port that they already control?
They had a deal about the Sevastopol port for 20 years. The deal was made in 1997 so the time was running out.
(
http://www.nytimes.com/1997/05/29/world/russia-and-ukraine-finally-reach-accord-on-black-sea-fleet.html )
Times had changed in those 17 years. The control that Russia had over the Ukrainian goverment disappeared when the rioters caused the president to flee to Russia. If things had calmed down then you can bet that Ukraine would have started moving politically away from Russia and in a few years they would have lost the port and any chances of getting a new deal would have been low.
Now they will are taking Crimea and even if they are unable to take the entire Ukraine they get to keep their port and strengthen their position in the region.
Also, had Russia waited to make a move, NATO, EU or UN might have moved into Ukraine to calm things down, by making the first move they stopped those attempts as no one seems to want to start a war with Russia.
EDIT: Russia is now having military excercises right next door to Finland... And their media has already had anti-Finnish propaganda...
...There are no actual threats against Finland yet, but I do not like where things are going. Someone already joked that few months from now, this will be concidered as the moment when you should have packed your stuff and run.