I really liked the way WoW did it at some point: When you earned XP at the level cap, it was instead simply converted to an extra gold reward. It wasn't an overwhelming amount, of course, but it did help to mitigate the sting. In addition, I don't think you even saw how much XP was converted, so after a while you sort of forgot about it, which was nice.

That said, with regard to challenge log achievements at least, I believe the game notifies you when you are about to complete the challenge. This is probably there to let you change to a different class if you wanted to.