Broken classes are like...
.. cheat codes. OK, I get full BAB, full spells, animal companion, good saves, why not throw in wildshape?
Posting the class here is a great measuring stick. If no one says "that class sucks," it is almost certainly overpowered.
On the other hand, players are often only motivated to add flavorful new classes, spells, items, martial arts, etc., if they have something to gain. There is nothing wrong with letting a new class be a good class, on par with say druid or wizard, as long as it is not insane compared to the other characters in your campaign. If one guy does it, it could inpsire other players to create their own classes as well. If you are willing to shoulder this burden as DM, you could end up with a very interesting campaign / party. Just make sure the players have RP reasons for the classes as well as power reasons.