Some people consider the way the lid closes (or doesn't close) on the Aluminum G4s a defect, but it's more of a slight design flaw, or a feature depending on how you look at it. I've seen a few of them where, for whatever reason, the keyboard was actuallyy making contact with the screen and scratching it noticably. Apple provides little cloth thingies to put in between the screen and keyboard, but I've never needed one on either of mine.

Earlier Alu G4s had dodgy latches that wouldn't keep the lid closed, haven't seen any of those complaints in a long time.

Nothing else, really. They're perfect machines. Go get one.