I always felt that for a product called webOS, Palm had one of the worst mobile browsers. The big missing feature I remember was lack of support for downloadable fonts. I don't remember the Touchpad itself being particularly polished (although the corded charger itself is supposed to be very high quality). Some of the touch stuff was pretty slick (the rubber band effect in particular), but a lot of it was pretty infuriating for me to use.

The mobile OS I miss is the most is the last iteration of BlackBerry. Although Palm had the upper hand in developer/user relations, I think that BlackBerry really got the UX right.