A cross-platform store is something that appeals to the consumer rather than the game developer I would think?

Definitely in my case - I play a lot of my Steam games on both platforms. Gaming PC at home, and Macbook Pro on the go.

I'm also of the opinion that the problems I've seen are too deep to realistically be fixed in the few months left before W8 goes RTM.

Why do they have to be fixed before then?

Because they're fundamental UI issues that Microsoft generally won't touch after a general release.

I thought we were talking about Steam which is made by Valve.

If you're using multiple platforms, I could see the benefit of not having to enter your credit card multiple times. But honestly, if you're going to buy a program on your PC, then walk over to your Mac and buy it again, do you really care if you're buying it from the same store both times?

I know that Steam lets you take your games from one machine to another, but I don't expect that to be the norm for most app stores.

This is not how that works -- if you buy a game for windows on steam, the mac version is free. I've tried this personally, and I expect the same will be true for linux as well. In other words, pay once, get the game for all OS', updating automatically, and all connected to the same account so you can chat to windows and mac players through the steam client