Nobody really knows because Windows 8 isn't out yet (sure, there's a developer preview and a release preview but no "official" final, and I doubt people have tested EVERY Steam game on even those "test" versions of Windows 8 yet).

To my knowledge, there are no big compatibility problems with Windows 8 that can't be resolved but it doesn't mean that any one particular game is guaranteed to work unless the PUBLISHER of that game says so (same as when Windows Vista and Windows 7 came out and we had to retry all our old games and either run in compatibility modes or perform workarounds or just hope for updates).

*Steam* runs just fine on Windows 8. Whether the hundreds of individual games by individual publishers work is purely down to whether they can be bothered to make them work on Windows 8 or not, or whether they have programmed them well enough to "just work". And buying the same game through Microsoft's online store? That will solve no problem that having the game on Steam won't solve. Either the *game* itself runs or it doesn't, nothing to do with Steam.

The simple answer? Nobody knows. The version of Windows 8 that actually goes out to stores could break every program in the world. We don't actually KNOW until it's out, and we try.

When Windows 8 is released (end of this week), I get a free upgrade for my machine. I'll install it, and tell people which apps I spot a problem with. But it might take MONTHS for me to get around to even running a particular game, let alone finding a problem with it. And if they are broke, it's the game that's broke, not Steam - so "buying games again" is not only stupid but pointless.

Once a steam game "License" for a "Subscription" to a game is tied to your steam account, you can download it as many times as you want on as many machines as you want while you are logged in with that account. No need to re-buy anything. If its yours now, it stays yours forever.
After you get each game downloaded then you're dealing with Win8's peculiarities (whatever those may be).

Also nobody is (yet) forcing you into Win8. I think you're giving in to the hype. The ONLY reason I moved away from Windows2000 was that L4D2 had a requirement for XP or later hard-coded into the game. I'm sure it would have worked fine if they took that OS NAME code out. I'm currently using Vista, but I could install NT if I wanted. Also Ubuntu Linux does an almost reasonable job at running most windows programs (although games support seems to need more work).