I agree with Billingsley. I think he pretty much nailed the major issues with the series. I've been looking at it again from time to time, and while I thought the first season was very weak originally and the characters were drab, they are faring much better the second time around, so I don't fault the writing as much as I once did.
I definitely see studio pressure being a problem and I have to say too that the old dreaded B&B creative fatigue had to be there as well.
It was time for a shakeup, some fresh blood. I wish Manny Coto had been the showrunner from jump. ENT felt stodgy and standing still compared to many of the other genre shows of that period. It really wasn't must see television in the way 24 or new BSG were.
Looking back, it was a solid show, with very good production values, and felt more faithful to the Trek aesthetic than CBS All Access Trek, but that wasn't enough.
The Suliban weren't bad villains but definitely could've been developed more. That said, I wish the Romulan presence had been there from the pilot to give old Trek fans the anticipation of the looming Earth-Romulan War. I also think Terra Prime could've, should've been a bigger problem in the series, and introduced much earlier. Paxton could've been a recurring villain, a dark mirror to Archer in a way.