People complain that ENT wasn't 'true Star Trek', but to me, it's a product of its time. I've not seen much to be fair, so I need some opinions. Say things like, was the writing good? Did it follow continuity properly? Things like that.
I think continuity was a big issue for many fans. Of course, the creators of any Star Trek production are going to find themselves stuck between a rock and a hard place when trying to reconcile a good story geared for a modern audience, and trying to maintain continuity with 40 years of Star Trek episodes and movies.
Enterprise concentrated a lot on the early relationship between Humans and Vulcans, which is a great idea in theory. But you have to realize that Spock is an iconic Star Trek character, and his father Sarek also has been a favorite character for more than a generation. People take this very personally, and wanted to see more Vulcans like them. Instead, Enterprise portrayed Vulcans primarily as arrogant and bigoted, even instigating a war with the Andorians rather than being primarily the logical and peaceful Vulcans we expect. We saw very little of the Spock-like Vulcans fans have come to know and love. It's one thing to try to add culture and depth to a race we want to learn more about, it's quite another thing to change them completely.
There were other continuity issues as well, such as the "forehead alien of the week" syndrome that other Star Trek series are known for. Basically, they explored new worlds and met all kinds of new aliens that we have never seen before. This is interesting because if the show is a prequel, most of the aliens we see should be ones we can reference from other shows. Since they haven't travelled far from Earth yet, where are all these new aliens coming from? Sometimes the show didn't feel like much of a prequel, it felt like an alternate universe.
People have also criticized the character development of the Enterprise crew, but I think this post is getting quite long.
I watched most of the first season, and sporadic episodes here and there since then. I really wanted to like it, and kept trying to get back into it, but it just lost the flavor and feel of the Star Trek I have always loved. I'm one who likes to consider it an alternative timeline rather than trying to force Enterprise into the rest of the Star Trek universe.