I'd like to see some interface changes. For one, give me faster menu interface. When playing a fast-paced multiplayer game where you only have 2-3 seconds between rounds, I'd like to be able to leave feedback, check the detailed achievement description, etc. Waiting on those blades takes forever!
I'd also like to see Microsoft step in and help control pricing schemes more. 800 points for solitaire? That's not even a maybe. There's no way I'd even consider that. On the other hand, I gladly paid 800 for SotN. I thought I got TMNT at a -cheap- price.
I've also been a fan of the free games that MS has given us. It saddens me that there have only been two live arcade games given for free thus far. Again, I don't feel like I'm just another whining kid begging for more free stuff. This last free one obviously was designed to promote XNA. But in addition to that, every time a valuable freebie is given (read: not advertisement gamerpics) it makes me appreciate the system even more. I can go to a friend that plays on a PS3 or Wii and boast about how I got a free T-shirt, an arcade game, and a retail game for free simply by playing my 360. When a company does things to make the customer feel appreciated, it makes me more inclined to buy from that company. Kudos to Microsoft for turning me into a true fan.
To that note, why not throw up some more challenges on the rewards site? We've already (finally) gotten the prizes from the only challenge up there. Give us another XNA game, or heck, I'd take an exclusive gamerpic for a small challenge.
Microsoft talked about the possibility of having an arcade section for non-localized foreign games. That would open up a world of classic games (pun not intended). Something like that would really make me happy.
I'd also love to see more XNA work delivered to non-subscribers (Especially considering it's a hassle to compile it on your PC, plus you don't get achievements.)
Now that the PC is getting achievements, why not give us WM6 users some arcade games? I'd give up my DS if I could get achievements on my PDA!