"Enterprise" seems to have improved this season - and I think it's because they've purposely pulled back from season long story arcs to multi-episode story arcs. Sustaining a single story line for an entire season is hard - even on shows I like, with generally strong writing (like Buffy) - it's possible to come up with entire seasons that are stinkers. In the BuffVerse, seasons 1 (The Master), 3 (The Mayor) and 5 (Glory) were the strongest. Season 2 (evil Angel) was ok, but 4 (Adam) and 6 (evil Willow) were pretty bleak - the overall story arcs simply couldn't sustain 22 episodes. That's where Enterprise went wrong with the "Temporal Cold War" arc - the writers spent most of their time painting themselves into corners, with the unsurprising result being that the final resolution was just silly. They are doing much better now with mini-arcs of 2-4 episode duration. The only issue I have with the current arc is the need to tie into the future that is already owned by earlier series (original Trek, etc) - there would be a lot more flexibility without that. I can hardly fault the writers for that though; it's a constraint that they simply have to live with.
Buffy was probably the best series that attempted season long arcs; "24" has to be just about the worst. Season 2 jumped the shark early on, and season 3 was just absurd by the end (you know it's time to stop watching a show when you are reduced to yelling at the TV). It takes a really strong author to sustain a long story arc - based on the evidence, I'd also say that it requires a single author (think Babylon V). "Enterprise" should fare better now that they've decided to stop trying that.