Doing professional iOS development (and hobby projects with the platform), I've been spending a minimum of 8 hours a day in XCode 4 and there are a lot of really big issues. My foremost grievances:

- A bizarre tab design that, despite the ~month I've had to get accustomed to it, is still compromising my efficiency

- An inability to cope with external modification of project files (e.g. update/merge will often break your project configuration unless you quit the app beforehand)

- Project files maintain their own fake directory structure (groups) when in nearly all cases you just want a 1-1 mapping to what you've got on disk.

- There are an abundance of pretty conspicuous UI bugs the Interface Builder component/panel (e.g. UI elements failing to draw)

That said, I'm a big fan of the humorous error dialog I get to see every couple of hours asking me if, after the application's encountered a critical exception, I'd like to "continue" or "crash".