At the time, people "hated" Enterprise primarily because of two things that I can remember:

1. It dared to be different in being a prequel.
2. Star Trek fatigue. By the time Enterprise started airing, Star Trek had basically been running so long in the form of TV, film, books, games and other media that it almost cursed Enterprise as being perceived as "more of the same", especially after what many consider a lackluster series (Voyager) and a couple TNG films that didn't get as much attention in theatres as was hoped.

Now it seems like we've begun to appreciate it a lot more in recent years. It's much easier to watch it from a fresh perspective now and think "oh yeah this did have some cool ideas". It really does feel like exploring strange new worlds and dealing with the problems in doing so (and it also retained that "classic" Star Trek cinematography/story telling, rather than the more fast paced contemporary renditions). It also throws in some fun tension in the form of the Klingons, Andorrans, the Temporal Cold War and other events unique to the series.

It's truly a shame they cancelled it as I'm sure that if it got 7 full seasons, we'd have got an evne more rewarding story and rich characters (regarding the latter point, many of them feel a little hollow). The best part to me was seeing the early Federation of Planets, namely our interactions with the Vulcans and Andorrans (and even Ferengi! although they aren't named).