Star Trek: Enterprise
Star Trek: Enterprise (often abbreviated ENT) is a roundly panned series that was cancelled after just four seasons instead of being allowed to achieve the "traditional" seven seasons that every Star Trek show has done since Star Trek: The Next Generation.
The Enterprise (NX-01) design was roundly criticized for looking too much like the TNG-era Akira-class starships instead of the older ships that had been seen from time to time in various Trek shows.
Continuity? What Continuity?
Another point of criticism was that the writers did not follow long-established Trek canon; instead making up history as they went along, often using contrived scenarios to justify introducing characters and concepts that had no place in a prequel.
Token Black Guy
Ensign Travis Mayweather soon became the series' Token Black Guy; he frequently failed to appear in episodes, and when he did, it was often to say a single line or just lurk in the background.
The Temporal Cold War
The Temporal Cold War was a recurring plotline about unspecified future people trying to alter history by messing with the past. The plot thread never really went anywhere, and -- damningly -- not even the writers knew how this plotline would be resolved.
The Final Episode
The last televised episode of Enterprise (and very possibly Star Trek in general), instead of following any story arcs that had been started earlier in the season, mostly featured TNG characters Riker and Troi, and featured them exploring a holodeck program featuring the Enterprise's adventures. The idea of the series' finale being devoted to characters from a completely different series pissed off fans and even the show's actors, and they were further angered when Tucker ended up getting a pointless off-screen death that a redshirt would be embarrassed to receive. Some have pointed to the holodeck program in the episode as proof that Enterprise is in fact itself a fictional show within the world of TNG-era Star Trek, which helps explain the many continuity errors in the series (although the final season had been making some effort to correct these errors).
Starting from the late third season, the show's quality began to improve, mainly thanks to a new producer and changes in the writing staff. The fourth season is generally regarded as the show's best effort at living up to it's premise, garnering praise from both the fans and professional reviewers.