Currently, and someone correct me if I'm wrong on this, but the following:
VB code support
Support for the enterprise environment including server explorer rather than just data explorers - this is for the entire VS environment
Outside of that I don't think that there is a ton different at the moment. I use Professional so I'm not sure all of what Express may be lacking.
I'm new to developing for the phone os, but am generally an ERP/BMS developer in VB.net. So for me I prefer to have VB, just preference from my perspective.
The express version also doesn't support add-ins. Makes it more of a pain to use with source control. Again though, that's more of an enterprise type thing, although there are some other useful add-ins out there.