It didn't take enough risks until it was far too late to save itself.

I didn't hate the first couple of seasons; they had quite few good individual episodes. But even the goods ones were mostly just forgettable. Unlike most, I enjoyed the Xindi arc. I feel like Enterprise was ahead of the curve in the shift towards having season long arcs. I'm not necessarily a fan of such things, but Enterprise I feel did a really good job of sprinkling in "adventure of the week" episodes while in the larger Xindi arc.

Where they really got going was when they found that sweet spot in season four where they starting doing two or three episode almost mini-seasons, and finally not being afraid to make their mark on the canon of trek by saying "We caused the flat skinned klingons" for example.

While that is kind of a dull example, prior to that, Enterprise almost went out of it's way to be unobtrusive, so-to-speak, so as to disrupt established canon as little as possible. If they had said "screw that...we're here, and we're diving into canon come hell or high water. Why? Because fuck you, that's why." It would have been a far more entertaining first two seasons.