I hear frequent watchings of TATV shortens your lifetime by two years.
But alright, I'll bite to your original questions. When ENT originally aired I watched season 1 and part of season 2 before I just gave up on it in disgust. The writing was just bad to average at best, the characters just failed to engage me and it was just a pale imitation to TNG, DS9 and even VOY that was disappointing in that it didn't live up to -any- expectations.
I eventually did re watch it on Netflix a year or two ago, and made it through the entire series. Seeing the Augment arc with Brent Spiner on SyFy got me interested in it again and lack of Trek for so long softened me to the concept of giving Enterprise another chance. I'll still rate it as weak compared to all the other Trek series, but since there weren't any expectations attached I can't say I was disappointed. Seasons 3 and 4 actually had some good episodes too. Had they started out that strong in seasons 1 and 2, I expect they would've retained more of their audience. Granted every Trek series had a comparatively weak first season or two, the market is less forgiving towards that than it was in the 90's.
When I first heard Enterprise got cancelled, I wasn't surprised and just shrugged remarking that show sucked. In hindsight now, I wish there had been a fifth season, though I cringed at some of the ideas that were apparently being planned like having a Starfleet officer be the original Borg Queen. Though at least it probably wouldn't have ended up on the stupidity that was TATV. I honestly don't know what they were thinking.
As for what I would've like to have seen? Drop all that Temporal Cold War nonsense. Seriously, I don't know what Berman and Braga's deal with time travel is, but they'll go with nine bad time travel episodes just because they'll occasionally get one good one. I thought it was bad how often Voyager time traveled to the point where the guy charged with monitoring the timeline goes insane and wants to kill Janeway for how often she screwed with the timeline, but then Enterprise makes it their one major story arc. It's easier to travel in time than warp from point a to point b in Trek.
Also character development. Enterprise apparently tried to emulate the "big three" from TOS, in form of Archer, T'Pol and Tucker. Everyone else basically got shoved more and more into the background. But those three really don't carry the show well. Having the secondary cast appear and develop more regularly and having a strong support cast like DS9 would've gone a long way. Again, by the time all the characters really started to find their selves in seasons 3 and 4, the damage to the viewer base was already done. You can have a show about the first deep space exploration ship without them all being a bunch of bungling and inexperienced oafs like we saw in the first two seasons. When NASA landed on the moon, they planned the mission down to every detail. Archer just seemed content to go "screw planning" and ride by the seat of his pants. He can be a strong improviser in unexpected situations without being a parody that refuses to do -any- planning though.
To the Earth-Romulan war? I'm actually glad we didn't see that in a small way. It still remains a scary and vague threat that way. Plus yeah, it would seem Starfleet is a number of years away of having enough ships to wage an effective war anyways if they're only turning out 1 NX class every 3 years.