Well you can buy all kinds of stuff from there now.
There's additional content for games like new maps for Halo 3 or new cars for Forza, even full blown expansion packs such as the Shivering Isles expansion for Elder Scrolls IV Oblivion. As far as I know there aren't any full games up.
There's also movies available now both in standard and High Def but they come with strings attached as they are basically rentals. You have (I think) two days to watch them before they expire, which I think is totally retarded but they're offering them at a pretty low price (Most are 250 MS points or about 3 dollars) so I guess it's not that big of a deal.
Then of course there are demo's, which vary greatly in size but the biggest I've ever seen was about 1 Gig. They usually rank in the 500-600 MB range, though. Oh and most importantly, demos are free.
There's also Xbox Live Arcade games, dashboard themes, gamer pictures and likely some other things I've forgotten. However, most of these don't take up much space at all (particularly the themes and pictures) and since the Arcade games are size capped at like 256 MBs or something like that, you'll never see one bigger than that, unless of course Microsoft ups the size cap again.