For my needs this is a no-brainer.
Personally a tablet would be indispensable if it could be the mac that I carry around to meetings, use on trains and planes and generally have with me to stay connected to my work and personal life without lugging my MBP around. Admittedly the iphone can do a lot of this, but in a small-screen size that's not suitable for professional/power use. On the other hand the MBP is brilliant, but doesn't connect to a cell network by itself, and is not the kind of device I can have with me at all times.
Features would include:
1. A-4 Screen (is that 10 inches?) that allows easy reading & notation of documents (inc word, pdf & jpeg: basically an e-reader on steroids) - I would use it to manage all my reading and briefing work, annotate maps and sync work server or dropbox - this alone would mean that the thing would practically never leave my side.
2. 3G and wireless, with push notification over 3G for email and staying up to speed on events (I work in emergencies)
3. Either native OSX applications or apps store (apps including news, radio and productivity - Omni apps and RSS readers such as netnewswire).
4. Microphone & camera for recording meetings & conducting videocons over skype etc.
5. Docking station that allows quick connection to a second screen, keyboard/mouse and external hard drive for backup (time machine, nats).
6. Mini-display port for running presentations.
7. Wireless sync with the other macs I own, and streaming music etc over airport.
Add to this the usual browser & lifestyle aps (dashboard, iTunes, iPhoto, VLC player etc) and I almost don't care what it would cost, but that's just me
