Thinner or not, QWERTY is a must, preferably as tactile as on the N900 but with more head-space of the top row (several millimeters would do just fine). However, if it does sport a capacitive screen as the rumors have it - count me out! I'm not compromising the precision and touch-it-with-anything (especially nails and stylus) features for some flimsy multitouch or feather-light touch registering - for the first there is a solution from Stantum, for the second - grow some f. muscles!

As for the screen size, 4" would be optimal, I wouldn't go above that. A digital compass and an useful hot-swapping microSD slot would really be nice, and I'd literally drool if it would have an USB host connector.