I've seen the arguments for and against.
I didn't like season three all that much, because it felt out of place. I followed the first two seasons, and saw Enterprise moseying along through the galaxy. That was great, I loved it. Then--BAM! We're suddenly knee-deep in vengeance and crazy Vulcans. It was sort of unexpected, we weren't prepared for it, and it caught us off guard.
Was that a good thing? Sometimes it is. But a radical departure from the format of a show is just that--radical. Enterprise does have a storyline, all of this Temporal Cold War stuff that we've glimpsed at.
So it's got two choices: become DS9, or become Voyager. Both had storylines. DS9's was more prominent, especially towards the last season. I have no problem with DS9, but it's tough to watch just any episode. Voyager, on the other hand, had a more casual storyline: get home. It was more comfortable, and that's why I prefer Voyager's storyline to DS9's.
I just thought that they overdid it on the whole Xindi-revenge thing. But I don't hate them for it. I haven't seen Zero Hour yet (I missed it!) but I'll catch it on Sunday. And then we'll see.
I'm all for a fourth season, I'm ready to give Enterprise another chance. But they really need to focus, because Paramount has put the Fear of God voice into Star Trek.
They gave Star Trek: Enterprise the Death Slot.