Star Trek: Enterprise began as a decent, interesting show, propelling Trek fans into a not so distant future, full of strange new worlds and alien creatures.
Season One began on a high. The two-part pilot episode kick started the new series with a great story, fantastic effects and a diversity of character's. However, this soon started to change as the writer's began adding familiar enemies and species from other series', breaking Trek continuity.
At first, there were subtle references to alien species that had been met in TNG, VOY and DS9 but these soon changed into full-blown inconsistencies. The likes of ENT 1x19: Acquisition, the Temporal Cold War arc, 2x02: Minefield, 2x23: Regeneraton, and the Xindi arc, sent Enterprise into a downward spiral from which it could not recover.
The series finally came crashing down with Season Four, which contained a plethora of contradictions and inconsistencies. The several story arcs - the Augment's, Vulcan Awakening, the Romulans, the Xenophobes - were poor last ditch attempts at reigniting the series and rekindling fan's interest.
Overall, Enterprise wasn't a bad show at all. The first season was pretty much excellent (bar one or two episodes), and the second season was very similar. In my view, the real problems started from season 3 with the Xindi arc, and from thereon Enterprise viewer ratings slowly declined, until, half way through the fourth season, the show was cancelled. As I said before, it had the potential to be a great show, had it lived up to established Star Trek canon and events.