Access to Crimea from Russia - Ukraine stands in the way. I don't expect Putin to invade, since I don't think he'll need to. He already has the Ukrainian gov't pretty well cowed into submission.
Access to Mediterranean from Crimea: Turkey has already threatened to close the Straits of Bosporus to Russian shipping due to the Crimean situation. That can't sit well with Putin. Turkey will be a much tougher nut for Putin to crack, IMHO.
“Turkey has already threatened to close the Straits of Bosporus to Russian shipping due to the Crimean situation. That can’t sit well with Putin.”
Turkey has its hands full with internal problems right now.
Turkey probably more than Western Ukraine. To retrieve Constantinople is a Russian dream since 1453