Several things for me:
1) I'd buy the full Win8, not the RT, so the ability to use all my regular apps (including games) is big
2) The keyboard. I HATE using virtual keyboards. Its fine in a pinch, but writing an email or post is painful. Having that keyboard as part of the cover is genius
3) Ability to play WTV file (Windows Media Center recorded video). I have a media center PC that we use as a DVR. I can use handbrake to convert stuff to play on my phone, but its annoying. Rather be able to just play the file natively
4) Ability to back up the tablet to my Windows Home Server
5) Everything else I own is pretty much MSFT anyway HTPC, laptop, desktop, Home Server, XBox 360. If they integrate Windows Phone 8 well, I may switch to one of those as well.