As I've done more code reviews at Justin.tv, I've started to notice a pattern. When I get a diff and I have trouble keeping focused because it's just that boring, this is a sign that the code is pretty much ready to go into production. This isn't to say that the code should be repetitive; repetitive code is not boring. I imagine it might be boring if I read it, but the instant I see the same pattern more than once in the code I just make a note to refactor it and move on. So a really repetitive code review actually is one of the more interesting ones to read - if you don't find noticing repeated patterns in code fun you're probably not a programmer. A really boring code review has very little obvious repetition.
Another thing that often makes a code review interesting is implementing a new generic abstraction. Personally, I love generic abstractions. New kinds of iterators are a particular favorite of mine, because they allow you to so cleanly separate what you do to each item from how you find each one.