6) Smallville
Prequel shows are often at a disadvantage as the audience already knows how it will end. As such, it can be difficult to know when to call it quits. This is certainly the case with Smallville, which basically ran five whole seasons too long.
Beginning with a teenage Clark Kent learning how to deal with his Kryptonian powers, Smallville was originally a fun retelling of the Superman story, just without the tights and flights. After season five, just about everything was in place for Clark to step up and become Superman. The show could have bowed out gracefully and it would have been gone down as a show that stuck to its original premise.
Unfortunately, Smallville ended up struggling on for ten seasons, introducing more and more elements from DC mythology to the extent that it was just plain stupid that Clark hadn’t suited up yet.