I'm not arguing for the majority here. I liked some aspects of Enterprise but the "Big 3" aspect was one that I didn't like. Trip probably grew a lot but that doesn't endear him to me as a character.For Trip and T'Pol, I wasn't only talking about romance. I was also referring to 2 people who initially didn't like each other and later formed a friendship. The point is that even some non-shippers saw good chemistry between the two.
To clarify, I liked Sato's development from wuss to a brave officer willing to do anything to protect her crew. It's just that the revelations of her past in Observer Effect felt forced. It was telling, not showing. For Reed, it wasn't as much of a shock to find out he was a spy since he was so reserved. But it still felt like a last-minute attempt to make him interesting.
As for Trip's development, the only way I can think of that it wouldn't be meaningful is if you take TATV as canon. But most of us don't. It's hard to believe that it wouldn't feel organic when he got so much screen time. I hear over and over again on this boards and Trek FM how much Trip has grown and they have nothing but good things to say about it. That tends to happen with arguably the most popular character on the show. So your opinion about him is probably in the minority.
Also, I thought Reed was interesting from the get-go, so the spy part was something more for me to enjoy. Again, mileage may vary.