I didn't care much for the Xindi arc but one reason I liked season 3 was T'Pol got rid of that ugly Moe haircut and let her bangs grow. She looked best with her natural hair in the mirror universe episode in Season 4.
The Xindi arc was a story I wasn't interested in seeing, but by and large (especially for Trek's first attempt at a full season story arc, as opposed to DS9 which just did stretches of episodes, and then others not connected) I think it was a very solid attempt. Especially for writers who had never done story arcs before at all.
I would have much rather seen a Vulcan/ Andorian war arc (or even something like a Andorian/ Tellerite war acr. But from reports at the time, the studio/ or network had asked the writers to cut down on Vulcan based stories (I think this occurred after season one, Kind of like DS9 first season was all about the politics of Bajor, and the writers were told by the studio to stop that, they then switched to religion and told to cut that down too). Something that would show Earth forces working to solidify some of the known early players in the Federation. I loved how in one episode ENT touched briefly upon the elements I wanted to see in Cease Fire in the 2nd season (as ENT was back on track story wise at that point).