For me, I had a bad taste in my mouth about prequels since SW:PHantom Menace came out a few years earlier. Like TPM, I guess I just wasnt really interested in 'origin stories'.
The opening theme song was atrocious and I had a really hard time getting past that.
I didnt like the ship design at all, and the CGI was piss poor. I know, it was the 90's/early 2000s, but I think there was way too much of a push to use CGI when it didnt look that good, especially when on a budget. If they had used miniature photography, I might have been better sold on it.
I just couldnt get into Scott Bakula as a federation captain. He seemed to soft and feely. And all I could think about was Quantum Leap. To no surprise, his old costar appeared along side him on Enterprise which further reminded me of QL.
Then there's the on-set tech. I knew it would happen and of course it did, the tech looked eons better than what TOS was and I found the continuity problem too jarring. I liked TOS, and so when watching Enterprise, it was like I had to forget that TOS and the Kirk movies ever existed. It was asking too much, I coudnt suspend disbelief and so I didnt like the show cause I felt like it was trying to rewrite Trek history. This is my biggest beef with prequels in general. Amazingly, SW:TPM avoided this problem successfully.
I also thought that Enterprise was the beginning of juvenile behavior of bridge officers. Not as much as what we see in NuTrek, but I could feel the beginning pangs. Maybe it was Tripp's character. Maybe it was the obvious sexualization of the T'Pol character, ie a repeat of Seven Of Nine. It just seemed like, intellectually, things had gone down a notch. That this show was trying to cater to pubescent teenagers. I always felt Star Trek had been aimed at a more intellectual audience until that point.
I hated the show after a few episodes and only checked in every once in a while to see how things were going. For me, classic Trek kinda ended with the conclusion of Voyager. And even in hindsight with NuTrek to compare against, I dont remember it very fondly. I see it as where 'the end' actually started.