Voice commands and its own TV guide are the big ones for me. There is no law issues here just effort. They would have to hammer out deals with Sky/VM and Freeview box manufacturers to get all the CEC commands working right.
The TV guide would just be a generic thing, and not tied to any vendor EPG. I don't know how it could be a vendor box EPG, with out some kind of set top box direct support.
I don't think any of the current boxes provide CEC. The CISCO hardware in the VM Tivo boxes doesn't provide HDMI CEC. So it would have to fall back on an IR buster.