Everyone here's noted things that I also loved about the series (uniforms, the look and feel of the series, the various character arcs), but looking back on the series there's one more thing for me.
Whenever Enterprise went into combat, I actually had to worry. It wasn't like TNG where the Enterprise D was usually the most powerful vessel/used diplomacy to simmer down a crisis/had Q to bail them out
. And while there were other situations where you knew the vessel was in trouble (U.S.S. Voyager facing off against the Borg, Defiant first engaging the Jem'Hadar for instance) from the moment Enterprise narrowly survived that early first season episode battle, I always thought, "Oh man, they could be in trouble!"
That all escalated with the Xindi crisis of course. I think it took me a few days to pick my jaw up off the ground after "Azati Prime".
Whenever Enterprise went into combat, I actually had to worry. It wasn't like TNG where the Enterprise D was usually the most powerful vessel/used diplomacy to simmer down a crisis/had Q to bail them out

That all escalated with the Xindi crisis of course. I think it took me a few days to pick my jaw up off the ground after "Azati Prime".