For me, that's because it tells a long-tail story really well. From the very beginning of the show they knew exactly what was going to happen later on, allowing everything to tie together really seamlessly.
Watching that all unfold and your understanding of it grow is just fantastic.
My only complaint is the whole S4/S5 debacle, because they got told it was being canceled so they had to wrap up quicker than expected, then got another season and had to re-open all the threads they just closed. Somewhat awkward.
As for the actors, I think they did a fantastic job. They're part of the reason I don't think a newer version of the show could work - you just couldn't have a convincing Delenn, Mollari, G'kar, or Sheridan like they did in the original.
And sure, the visuals of the show aren't fantastic. But who cares? A good show's merits aren't in its visuals, so long as they're sufficient to tell the story.
People say the Sopranos ushered in the 'golden age of TV' and I woudlnt deny it had larger mass appeal, but shows like B5 did it first.
But what a disappointment -- some actors can act well without direction, some can't, and B5 really shows you with stunning clarity which side of the line the actors are on. On top of frequently stilted dialogue (a JMS soft spot -- his screenwriting book is great but his dialogue examples are terrible) and missed opportunities for humor -- and with those aliens and that dialogue, you'd better take those opportunities if you don't want the audience to take them for you.
The production values have also aged spectacularly poorly, but they probably looked decent on broadcast standard-def TV at the time and it's really not the primary issue.
Which is to say, they can't really do worse than the original with JMS himself showrunning it again. Maybe it'll be a lot better since they have more money/support this time.