I think the major problem that many people have with Enterprise is that it didn't really take advantage of its unique situation. So much could have been done with the setting and time they were put it in. Instead TNG-style episodes were largely run during the first 2 seasons. A temporal cold war? I think a lot of people were tired of time travel already. But even that could have gone over well if it had been handled correctly. Too few episodes were done on the that long-running arc, and as a result I felt it dragged out.
Season 3 and 4 really did take some risks and do something new. I liked the Xindi arc. Season 4 was one the best seasons of Trek. This new direction came a little too late though, as Enterprise had already bled out too many fans. It's a shame though, as season 5 could have been spectacular.
I agree. ENT certainly wasn't a completely awful series, but it did miss a lot of the potential that was there. And it's saddening to me, because I think that potential could have led to some good or perhaps even great stories. I'm not of the opinion that just because a prequel fails, the prequel concept is entirely bad. It's no different in failing to meet the standards of the original than a bad sequel is.