The revamped notification system will be very nice, and since I've most recently come from a BlackBerry iMessage seems really nice as well since it reminds me of BBM.
I second that, in the same order, while including photo stream in the camera app improvements. The new notifications are GREAT. I don't know anyone else running the GM yet, so haven't had a chance to try iMessage yet, but looking forward to it replacing Kik.
Notifications for sure! I switched from a blackberry over a year ago and really missed the notification system. I still think iOS sucks at autocorrect compared to the BB. I hope autocorrect gets addressed eventually.
iMessage will be good. I'm glad it seems to be integrated into the Messages app as it used to irk me on my old Blackberry that my messages were in two different places. I'm also excited for the camera improvements. Does anybody know if you're able to create photo folders on the actual phone itself yet? That'd be fantastic.