To add to 2), most games also back up your saves via Steam Cloud now. Not necessarily something you'll always use, but after a reformat, I greatly appreciated it.

However, I have noticed that Steam's offline mode is rather fickle. I try use it when I'm at my friend's house (lack of wireless and a lack of an ethernet port. Weird, unusual scenario, I know), and half the time "Play offline" simply doesn't work. It's put me in the weird position of considering downloading cracks for games I legally own so that I can play them.