I didn't like ENT's portrayal of Vulcans. I thought they were made too antagonistic, and always at the expense of making the human characters, particularly, Archer look better. That being said, I also didn't like how humans were portrayed. Though they had far less space experience than just about any other race featured on the show, humans were always right and human values predominated.
Personally I didn't want T'Pol really exploring her emotions. We've seen that before to some extent with Spock and Tuvok. Or at least the conflict between Vulcan logic and emotion. Why couldn't a human explore, or find value in Vulcan logic for instance? That's something we haven't really seen before.
T'Pol's additions to the logic v. emotion debate were muddled to me. For one, the whole addiction thing was stupid. Why would she need to take drugs to experience emotions she already has? All she would have to do is let down her mental guards. Also, I didn't really care for the mind meld being a dirty thing issue, though the Vulcan Reformation arc thankfully cleaned up a lot of the damage I felt ENT had done to the Vulcans. Though to be fair, I noticed this skewed portrayal of Vulcans as aloof, arrogant, and antagonistic first on DS9, "Take me Out to the Holosuite".
As for the rest of the cast. I don't think Archer was a well drawn out character. They had him mainly fighting his dead father's battles, and not being his own man at first. But in Season 3 he really grew as a character and I finally could really see him being a credible commander. Unfortunately, all of the characters took a back seat to plot-driven stories in Season 4 and we didn't much more development from Archer except perhaps in the episode "Home".
Trip seemed to become the focus of the writers and it allowed his character to shine the brightest.
Hoshi, Travis, Phlox, and Malcolm were woefully underdeveloped, Travis and Hoshi the worst of all. With Hoshi, they pinned this bad girl, martial arts thing on her in the fourth season out of the blue, and Travis never got beyond the deer in head light's characterization from the first season.
So, characterization was an Achilles heel for ENT. Especially in an age of drama where compelling characters like House for example were drawing people in. ENT's characters were bland. Even compared to the rest of the Treks, there was very little about ENT's characters that stood out. Which is a shame because the prequel format gave the writers a chance to do something different, to show us less polished characters. To some extent I do think the writers tried that, but their attempts were so-so.
That being said, I liked Seasons 3 & 4 of ENT during its initial run. Now, I no longer revile Seasons 1 & 2 as I once did.
