Maybe this is common sense, but I'm going to post it anyway because I had no idea.
Trying to launch MW2 this morning, I kept getting a 'This game is temporarily unavailable, please try again later' error. I have a kid, am a full time student, and have a full time job. I can't guarantee a 'later'. So I searched online for answers, which of course were: restart Steam, or restart your computer. I did both, with no change afterwards. I tried to launch other games, which worked fine, so I knew problem had to be with the game files, but Steam wasn't being too proactive when it came to fixing this for me.
So toying around, I found that if you right click on the game under 'Games' tab, go to Properties, then to the 'Local Files' tab, you can choose to 'Verify the integrity of game cache'. After doing so, sure enough Steam found some corrupt files and replaced them for me, and everything works fine now.
Just thought this may come in handy in the future, if not for somebody now.
Also a fun trick with the verify thing: If you're having trouble getting Steam to update one of your games, 'say the update just launched that day or a few hours ago' you can navigate to the steamapps folder, find the game folder you're trying to play and delete some files, I usually just delete the main .exe. When you go to launch the game again steam will force the game to update and re-download the files it needs.
Used to have that trouble when Half Life 2 first came out. Deleting the blob file usually fixed most of Steam's launching problems. Luckily like you have said there is a file integrity check now so it fixes most of the problems.
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