You could fill pages with things that can be improved on Android Market (most of which are available on the App Store). Most shortcoming are well documented - things that I would like to see personally:
The market client also needs some improvement: for example, the search feature isn't very good. Searching for "ADW Launcher" brings up countless pages of themes for the ADW Launcher before showing the actual launcher app itself.
- ability to publish what changed in an update
- track number of users who upgraded vs new users
- ability to search for apps and see reviews on http://www.android.com/market/ (that website is practically useless)