I really only want two things that my N800 doesn't have now:
1More horsepower. I don't know if it's software or hardware, or both, but we're not quite to the fie on a mobile device. The it does it better than any other device, but still can't handle some of the 'Web 2.0' stuff that I need it to do(Google Spreadsheets anyone?)
2Always on connectivity. WiMax sounds great, but it ain't here yet, and probably won't be cheap at first, either. I personally want a quad-band GSM sim slot with at least edge. I don't care if it can handle phone calls or not. I could move my current card with $20.00 a month data plan to the tablet, then add a line to my AT&T Family Plan for $9.99 for my calling needs. Cost to me:$9.99 per month more than I'm paying now. Not bad for full-time tablet connectivity. I know, the argument against this is that Bluetooth tethering will do the same without adding an additional line, but I'm not satisfied with Bluetooth. I've been using tethering for a few years now with several different phones and mobile computers/PDA's, some with built-in BT and some with dongles, and I've never been satisfied with the results. I've had too many dropped connections and other problems, so I'd rather take the HTC Shift route.

Beyond these two things, I'd personally stick to SD cards for the increased storage, but would at least prefer microSD to mini. Mini just hasn't had widespread adoption as far as I can see.
Finally, always give a pure tablet option. I'm not a big fan of little keyboards, but I would like to see 'write anywhere' handwriting recognition. I love handwriting recognition, but the it's current implementation is poor.