Features:
Phone Audio + Voice Control (Answer, Mute, Call, Dial-a-person) --- I believe these are pretty generic to the BT profile(correct me if I am wrong)
Phonebook Sync (again generic BT)
Audio Streaming including control (Pause, Next, Previous, Play) either onscreen or voice control
Audio details (album cover, artist, song name, etc.)
Support for a variety of players. I know many head units include Pandora. I do not need it built in but at support the above mentioned functionality via USB or BT. I was using Spotify (waiting for a WP8 app). Until then been limited to Xbox Music.
The last and I doubt this is currently possible, app sharing. Something like the Pioneer AppRadio 2 offers for the iPhone (Apps I have installed on my phone works via the touchscreen head unit, like navigation). Even then this required using the 30-pin Apple cable.
I understand what you are referring to as seeing the WP8 as a USB device. My current unit does not do so but that would be nice if I wanted to transfer music from my phone to the head unit's HDD (if applicable)
Pretty certain we all agree, the iPhone has changed this market. From just using AUX and listening to audio to a full control complete feature set head unit in our car. I really like some of the things that the Ford Sync appears to offer (which apparently) works with Windows devices; but then again I don't have a Ford.
I am using the OEM 2010 uConnect (same unit most of the Dodge vehicles include) in a Jeep Wrangler. It is a 2-DIN touchscreen with Navigation. The BT is limited to phone support (no Audio Profiles were available in that model). My unit is just a few clicks from becoming dead. Phantom channel and volume changing, touch screen unresponsive, etc.). Otherwise, I would just wait until I started seeing radios that stated Windows Phone supported.
Thanks.