Personally, I think Enterprise gets an unfair rap. I think B&B correctly pointed out that it was canceled due to Trek fatigue more than anything. I think Enterprise, as a show, gets a solid "B+". True there were some issues with it, but that mostly related to how they handled T'pol's character, for me.
Some complaints people have, as I understand them, are as follows:
1. Well, they didn't call it "Star Trek" in the beginning, and tried to downplay it was part of the Trek universe to gain a bigger audience.
*My response: So what? Only trekkers were going to watch primarily, anyway, and they fixed that by season 3.
2. They had a crappy opening song.
*My response: So what? do you watch the show for the show, or its opening theme?
3. They stepped all over continuity!
* my response: only in the novels preceding it were there serious violations. and they aren't canon, anyway. I defy anyone to point out anything that was "not canon" if you go strictly by the TV shows or movies, that is any worse than previous show canon violations.
4. I don't like how the Vulcans were portrayed or how they held the humans back!
*My response: I thought the way Vulcans were portrayed was absolutely fine. They had to show them with flaws, otherwise if Vulcans were perfect, why wouldn't humans just adopt the Vulcan way? Also, I thought it added to the Vulcans first being the teachers of humans, then finally learning from them and accepting them as peers.
5. The Xindi arc was not needed! i wanted the Earth Romulan war!
* my response: can't argue with that too much. I agreed with that, to an extent. But I liked how well the Xindi arc flows. It is the best arc Trek had, since DS9. Enterprise was trying to develop its own unique "enemy", I think. Also, from the standpoint of showing humanity is still in the transition of "old world" thinking and the later "enlightened UFP way of thinking (IE Prime Directive), and humanity was still attempting to mature. In the end, Archer and humanity use diplomacy more than anything to end the Xindi threat. not to mention, we surely would have seen the Earth-Romulan war, had Enterprise not been canceled.
6. The temporal cold\Suliban war idea was dumb!
*My response: Well, they had to do something so that the outcome of the show was in question. But I agree, it could have been written a little better.