4. Mail filtering. Still on the email front, iOS 5 does not support message filtering, such as to automatically trash all emails tagged as junk by your server or to move all emails from your boss to a specified folder. iPhones and iPads certainly have the processing power to do this, and Apple has such filtering tools in its Mail client in Mac OS X. At the very least, you should be able to sync those rules from your desktop (from Mail, Outlook, or Exchange) over iCloud or other mechanism, and eventually create and edit them on your iOS device.
5. Undifferentiated universal inbox. My last email complaint: When looking at the All Messages view of your inbox, iOS doesn't indicate which mailbox each message is from. It really should, so you can more readily see work email than personal, for example. Even better would be an option to hide some email accounts from the universal inbox, such as the account you might use for e-commerce and its accompanying spam mail. After all, Calendars can visually distinguish accounts for calendar entries and let you choose which accounts display in the universal view; why not Mail?
6. Voice-based search. Apple has rich assistive capabilities for the visually and hearing-impaired in iOS, which iOS 5 bolsters. So it's odd that iOS doesn't let you search the Web or the mobile device's contents via voice -- especially on the iPhone, where you are likely to be using a Bluetooth headset -- as Google's Android OS does. (You can do limited functions via voice, such as control the music player and call people in your contacts list.) Although I'm not a fan of talking to or hearing computers when working, I see the need for cellphones in some circumstances. Yes, the new iPhone 4S adds the Siri "personal assistant"-style voice-based search agent, but it's limited to just that one device, so I can't really count it as an iOS 5 capability.
7. iOS-only use of FaceTime and iMessage. I understand Apple's desire that everyone use its devices, and thus the impulse for making services like FaceTime videoconferencing and the new iMessage text messaging work only among iOS (and Mac OS X) users. But that approach greatly limits their utility. iOS 5's new iMessage client is rendered particularly useless due to this limitation: You might as well opt for email instead to be sure you can reach anyone you want (as opposed to using on the iPhone, anyhow, the carrier's high-cost SMS, an evil money grab that should be outlawed) or a third-party IM client that works across services (as most do these days).