More and Better Free games. This is where Apple and Google one up Microsoft. The Trial versions are nice, but there really isnt many free games that are fun.
I really think they need to make use of the xbox. Thats Microsofts biggest moneymaker right now and its very popular. Some kids are so crazy about it they would buy the phone just for it. Idk what the possibilites or capabilites are so I wont speculate, but any improvements/integration with xbox would be great. Something simple as turning your phone into a remote to control it while watching netflix would be neat.
For regular apps just more big name company apps would be nice. Stuff like USA Today, Netflix, Facebook, big companies like that its nice to have apps for. I love the 3rd party company apps dont get me wrong.