"This looks like a good case for a binary tree but it doesn't scale well under load."

"We would add support for this motherboard that looks a lot like the one we do support, but it doesn't offer feature X."

"We want to add more detailed logs here but we haven't figured out how to cope with the performance hit."

Closely related, comments are also useful for explaining why code that looks wrong/poorly optimized/obsolete isn't actually, which is something that the code itself can't explain.