Steam's cut is 30%. However, for that 30% you get to update your game whenever you want, and virtually as often as you want. By that I mean they don't impose any direct limits on how often you update, but the general rule is no more than once a week aside from hotfixes and the like. I'm not aware of any digital marketplace that gives you this much control, especially as video game updates will often be hundreds of megabytes in size. Contrast this with Microsoft's Xbox Live Arcade, which charges you $40k for each additional update after the first free update. [1]

Edit: It is worth nothing that if a user's game was activated with a retail key, Steam takes no cut. This means that you are effectively getting service for that user for free.

You also get Steam Cloud, which allows you to store your users' configuration and saved game files to be accessed anywhere, their peer-to-peer networking API which provides NAT punching. Then there's matchmaking, stats/achievements, Steam Community (their social network), and several other "Steamworks" features. [2]

Edit: Microtransactions is another big one, also known as in-app purchases. The online brochure for Steamworks is worth a read if you're interested in any of this: http://www.steampowered.com/steamworks/index.php

I haven't read a lot about Windows 8's app store, especially concerning Xbox Live, but Steam still has a lot to offer to developers.

[1] http://arstechnica.com/gaming/2012/07/microsoft-comes-under-...

[2] https://partner.steamgames.com/documentation/api