In particular, Calendar's entries in Notification Center can be sub-optimal. Like any of the other apps, you can choose to have one, five, or ten of your Calendar events displayed. Obviously, Notification Center simply picks the next upcoming events, but it would be nice if there were some more specific controls in place. As someone with a lot of work and personal calendars to juggle, I find that Notification Center fills up with calendar events that I don't really care about, while events that are more important to me get lost in the shuffle. The ability to have Calendar notifications pick from a specific calendar, or ignore certain calendars, would really improve the usefulness of Calendar in Notification Center.
One nice touch, though, is that if you have a reminder or calendar event that's keyed to a particular time, the time remaining until it (or after it) will continue to update on the lock screen.
iMessage, you message, we all message!
When Apple first released the iPhone, there was a lot of speculation about why the company didn't include an instant-messaging app. After all, it includes its own IM client, iChat, on the Mac. The prevailing opinion seemed to be that including an IM app would have detracted from one of the wireless carriers' cash cows: text messaging.
With iOS 5, Apple has evidently decided that its position is no longer as tenuous as it was back in 2007. In this update, the company has introduced a system called iMessage, which lets you freely send messages—text, image, video, a location, or contact information—to anybody else on an iOS device, free of charge. That's because iMessage sends its payload over the device's data connection (Wi-Fi or 3G) instead of over the control channel of the cell phone's voice connection, as SMS does. And because you're already paying for a data plan, those bits and bytes will only count against your cap for that, rather than against your SMS limit.
In essence, that makes messaging virtually free—as long as you're conversing with a fellow iOS device owner.
What's clever about how Apple implemented iMessage is that it built the feature right into the existing Messages app. So there's no having to explain to your less technically savvy friends how they can send you a free message instead of an SMS; it's all done automatically.
In its traditional attention to design, Apple uses subtle cues to differentiate between sending an iMessage and a standard text message. When you add a contact whose phone number and/or email address are associated with iMessage, their contact bubble and the Send button will appear in blue; if you're just sending a plain old text message, they'll be green instead.