full integration into every single xbox 360 game made meaning you can join games your friends are playing - steam can only do that on source games.
joining an xbox party automatically gives you voice chat that's clear - you need an extra program besides steam, about 15 minutes into calibrating a mic, and a button press each time you wan't to talk for on pc.
invite other friends or your whole party into playing your game is very trivial
when the inevitable death of xbox live comes in some years for whatever reason, i'd still be able to access my games since they're on physical media. if steam goes away tommorow, what will happen to your games?
when i do buy a game on steam, i take the cd-key and install the game using an iso. i don't need to advertise to the world that i'm cool because i've played bc2:vietnam for four hours last night. which doesn't really make sense because you can't join your friends bc2 game from steam and must use the HORRIBLE friends system on bc2 anyway.
truth is steam is nothing beyond source games. beyond the facade it's an adware/download manager for gaming that charges the same price for a boxed version.
i love xbl but it's a closed system along with xb360. it will never work with pc's open platform.