One of the touted features of the 3.0 iPhone upgrade was the addition of landscape mode throughout most Apple applications. Initially this sounded like quite a boon. Until I realized: I never use landscape mode, I don't like landscape mode, landscape mode is always activating when I don't want it to and there's no way to turn it off.
My biggest complaint about landscape mode — and what seems to me the biggest UI gaffe — is that it forces itself on the user. If you happen to be reading something in Safari using portrait mode and, for instance, lying on your side, the iPhone will constantly attempt to reorient the screen to landscape mode, even if it's completely inappropriate. It interrupts what you're doing and requires all manner of acrobatics to keep the thing in your orientation of choice. In the end you give in and just sit up. And that sucks.
The one time I almost always like landscape mode is when I'm looking at photos in the Photos app. Ideally we'd have a way to opt-in to landscape mode on a per-app basis. But at this point I use it so little I'd settle for a way to just turn it off.
I understand a lot of people love landscape mode, but the way it is now is far more annoying than useful for me.