I always felt Enterprise as a child of conflict. It wanted to be "retro" but it wanted the convenience of many Next Generation plot devices, technologies, and tropes. It wanted to have story arcs, but wanted the ease of stand-alone episodes.
Over the course of the first two seasons, there was the occasional episode that seemed conceived with the show's core premise in mind. Those were sandwiched between three or four episodes which were as stated above, rejected Voyager scripts.
The problem with the third season Xindi arc were twofold IMO: 1 the show should have been bold from season 1 and entered the modern era with full arc based seasons from the outset. 2. the Xindi concept, while actually daring in many ways, was mired in the temporal cold war bollocks for its basic premise, and didn't have much to do with the core concept of the show.
As I saw it, Enterprise was supposed to be about examining a very big idea. How humanity could evolve from 20th/21st century psychology, society, and ethics, into the 23rd century "evolved" humanity of Star Trek. Humanity in Star Trek has really put its big britches on. Humans there are so evolved, that they've not only solved all their internal problems, but become a role model to the galaxy and united other civilizations in a common cause.
In the same way that the original series used science fiction as a platform to shove uncomfortable and controversial topics into the face of timid and conservative audiences, Enterprise was the perfect opportunity to recreate the spirit of the original Star Trek. There's a wealth of material waiting to be mined with bold predictions and hypotheses about how human civilization might cut free the dead weight of its varied psychoses and grow to the next tier.
I think my disappointment with Enterprise crystalized in an early episode where a female crew member gets romantic with Phlox because she's got a crush on him. He explores this, and we see the human woman deal with working through some weirdness of his alien nature. And in the end, it falls through - why? Because Phlox clarifies that his society encourages polyamorous relationships and Phlox has multiple partners. And this human woman of the 22nd century freaks. Because she says, that's not the human way of life. Confused and grossed out, and backs away in humiliation. Phlox makes a pithy note to self that humans need to evolve a bit more.
A more daring script might have it that rather than be surprised and grossed, the woman takes it in stride, but informs Phlox that she's not poly; perhaps note that it's too bad he hadn't met a relative of hers who is, they'd have hit it off great. About then I wondered if Enterprise would deal with revealing a crew member was gay by making a big point about it and have Archer pat said crew member on the shoulder, informing them in a fatherly manner that nobody would hold it against them.
I think this has always been the failure of Star Trek in general past the original series. Nobody involved with Trek after Roddenberry really seems to have grasped what Trek was used for in the beginning. So while much of Trek and fun and cool and I have nothing against it, it's kinda represented great wasted potential, given that Star Trek is to America what Doctor Who is to the UK. It's Everybody's Sci-Fi and as such a great platform for ideas.
Sorry, was just thinking about the big Trek picture while browsing the thread.