I consider Enterprise to be a "Thinking Person's" series. It requires a little thought to go through the series and really get the point of many of the episodes. My feeling is that those who "hate it", either don't want to or are not capable of putting the thought into it. I mean really! What the heck is there to hate here!
Most of the so-called "hate" is simply disappointment. The show
should have been better than it was. Taken in isolation I'm sure it's not that bad; but in the context of the time it was airing, there were problems.
As an example, take the ship's weapons loadout. They started trying to do something interesting with the missile-like things and the grappler, but made the mistake of having them be
too wimpy; necessitating the replacement of those with (slightly renamed but dramatically equivalent) phasers and photons. Stuff we've seen before.
Context: All this while on Scifi, Stargate SG-1 was featuring a starship armed with nuclear missiles, railguns, and a few (captured) energy weapons. In many cases the armament doesn't make a whole lot of difference, but at least
they were trying something different, while ENT lapsed back to the familiar and trite.
Then there's the captain. We're promised a "Han Solo" type, and instead we get a daddy's boy who shows little if any sign of actually deserving the command his father got for him. Which would be fine if that were the narrative intent; but it was clear from the start that the writers
thought Archer was a lot cooler than he was, and that disconnect was patently obvious to the viewers.
Context: Meanwhile on Fox, Firefly was introducing us to an
actual Han Solo type, and the contrast just makes Archer seem even more lame by comparison.
I'm not saying it was all bad. ENT did bring us some great characters like Trip; I always enjoy when Connor Trineer shows up on other series for a guest spot. But there were definite problems, and despite being pointed out early and often, little was ever done by the creative team to address them.