Star Trek: Voyager
It had SO much potential for great character based story-lines. It had so many great back-stories for its characters...but it NEVER used them to their full potential. Instead it became the Close-Talker-Captain Show, and when ratings started to slip (because people like me had lost interest), they "solved" it by adding a model in a skintight bodysuit. Ugh.
It had it's moments, but it never made me care enough about the characters to watch. Basically, it had a great Pilot and absolutely no follow-through.