Just to be clear, while I think this situation is a lot different than Georgia and Crimea in terms of scope and stakes, which may impact the severity of sanctions that are ultimately levied or the resolve of US/EU/NATO members, I understand people's skepticism based on that history. Its put up or shut up time for the west.
What I dont really see is a path to a Russian-puppet government in Ukraine that doesn't inflict a large amount of damage in terms of blood and finances for Russia. It's one thing to claim South Ossetia, another to claim the 36th largest country in the world, especially where most of the country hates you.
Iraq serves as a great example of that IMO.