First of all, I wanted to say that I consider myself a moderate Trek fan. I've seen most of TOS, the first several seasons of ST:TNG, and most of the theatrical films, but I've never seen an episode of "Voyager" or "Deep Space nine".
That being said, I saw all four seasons of Enterprise on DVD several years back and was truly blown away by the show - I felt that it was extremely well-done and interesting. The characters were likeable, the acting was solid, the scripts were exceptional, and the effects were excellent, even by today's standards.
My favorite Trek film was First Contact, so I really liked how this show tied into that film. I actually prefer Enterprise to TNG and TOS & thus it's my favorite ST series, though I know I'm not in the majority here. It's possible that since I'm not a hardcore Trek fan I was more open to enjoying the show, since it was so different from the previous series.
One of the many things I liked about the show was the setting - it was set before the "Prime Directive", which stated that you couldn't interfere in a planet's doings, so the crew could have interaction with a planet's inhabitants. I also liked the show's homage to TOS, since it featured some of the older aliens like the Andorians (the blue-skinned ones with the antennae), as well as the Romulans.
I also thought it interesting that the characters were just getting to know and understand the technology that was considered standard in the later series, i.e. the transporter, phaser, etc.
The time-travel subplot (with aliens from far in the future) was very well-handled, as well as the storyline where they went back in time to WWII Earth.
There were also many, many stand-alone episodes that were great, including the pilot ("Broken Bow"), the time-travel one where they went back to 1950's America, the episode where Archer met a ghost-like woman on an isolated planet, etc.
I'm also probably one of the few that liked the intro song and images - it was cool how they showed the first airplane and space flights, etc., all leading up to the flight of the "Enterprise"
I would have to say that the only episode I not only didn't like but hated was the final one - I thought it was ridiculous that the events in the show were being viewed through the holodeck by TNG's characters (especially since I was never a real fan of TNG). It would've been much better & IMHO easier to just have it run as a regular episode.