The Icon/929 is much faster than the 928. Where the 928 may stutter or pause during a swipe or touch, the Icon never does. The screen resolution of the Icon is considerably higher and it's quite obvious. Neither is, to me, the most comfortable to hold but they both feel bullet proof. I love that about Nokia phones and I hope (but doubt) that Microsoft will continue that build quality. Although I may be overly pessimistic - Microsoft hardware is always excellent. In terms of sound, they both sound terrific. There are some reported problems with Wifi on the Icon, but I have never encountered that. Battery life is quite good, although KitKat has allowed Android sets to catch up and sometimes surpass it. I do like having a dedicated camera button. And speaking of the camera, the Icons camera is amazing. The 928 camera is excellent, but not as good as the Icon.
As for the 8.1 preview, I *THINK* but am not entirely sure that you get an additional column for tiles on the Icon, since the higher resolution screen effectively gives you additional real estate to play with.
Transferring to the Icon ought to be completely painless. My 928 was kept backed up, and as soon as I logged in to Microsoft, it just loaded up all my apps and recovered all my contacts and texts. It was like I never left home, with the exception of having to re-layout my home screen and the faster execution speed of, well, everything. WP8 is, imho, the best OS out there at the moment and it's even better on the Icon. I just wish they'd hammer out some of the 8.1 bugs - which may require a firmware update as well as debugging the OS. The only thing I wish they would do is allow some tasteful alternative to the constant flipping over of live tiles. The thing looks like a Christmas tree sometimes with all the bright blinky lights. Sometimes tacky.
The Icon is evolutionary - there are no operational changes with exception of the loss of Glance, and I don't think anyone here knows exactly why. The screen tech is the same I think, so as far as I can tell it doesn't have Glance just because it doesn't have Glance. One does not question the mighty Nokia, I guess.