Below are some iPhone issues I found to watch out for. Maybe take a minute (or five, like I did) and go over it with a fine-tooth comb to see if you notice anything "not quite right."
1. Dead pixel (After about a month of use)
2. Dust behind glass
3. Stalled in "Restore Mode" when I tried to cancel a software update
4. Small dent in aluminum frame
5. Glue protruding from paper-thin gap at the edge of glass
6. Glue on aluminum frame (easy enough to rub off, so I did)
With each new phone came a new challenge. Most issues I found with mine were aesthetic, but if I'm paying $300 for a phone, I want that thing to be perfect.
I'm on iPhone 4 #6! Apple has been very gracious though this whole process, especially Eric at the Aspen Grove store here in CO. No questions, just swapped it out--twice. :-)