Having played with Apple's iOS5 for a couple days, here are two big thoughts and a few brief ones after that: 1. This is huge for Twitter. I can't understate how cool AND how meaningful the integration is. The phone is effectively hooked directly into Twitter such that your content is always shareable. As Twitter focuses on engagement - this is a remarkably powerful step forward. And - it is done beautifully. The animations are gorgeous and the sharing happens seamlessly.
2. iMessage is equally impressive and important (just ask the carriers and the WSJ). I grasped the importance of iMessage from the WWDC announcement - but I didn't appreciate the user experience. When you messaging friends, it automatically / seamlessly transitions between SMS and iMessage based on their platform / phone number. The animations are great and the implications are broad (iOS product extensions, carrier monetization, SMS future, messaging landscape, etc).
3. Drag and drop calendaring is great.
4. Notifications are obviously game-changing to the consumer experience. For developers, this is a big opportunity to improve usability and engagement opportunities. For instance, ESPN Scorecenter is an entirely new application experience for me (and far more useful).
5. Small email tweaks are welcome (flagging, search, etc). Same with the mobile browser.
6. iCloud has gotten tons of attention, and rightly so - game changer for content purchasing and management.
7. There are so many new functions / enhancements that it is quite difficult to fully figure out usability. I have several Apple devices and this has been the most overwhelming transition yet. I'd go so far as to say it is mildly frustrating!