However, to expand upon your questions, I'd say elements like this:
- the advantages to community building, compared to the power of the individual.
- extended story arcs encompassing long periods of time
- uniting against darkness
- "not everyone is exactly what he appears to be"
- eminently quotable lines
- extensive use of CGI
OK, it's now clear what you mean. You're referring to both production elements and themes in the story. Here are several more ideas:
- Some attempt at realistic physics (space station needs to rotate in order to produce "artificial gravity"....Starfuries obey the laws of physics, and don't do physically impossible banking maneuvers in space). Although there are certainly counterexamples, in which there was some creative license taken with science......Crusade actually went one better in the realism department, and showed <gasp> the plane of the Milky Way in some of their space shots, which I've never seen done before in any other TV show or movie.
- Heavy use of what one might call "modern" directorial touches, such as frequent voiceover narrations, montages, and black and white flashbacks. (I love this style, and it was really used quite heavily from about late Season 2 to the end of Season 4.)
- Alien religions (and human religions) actually shown to have at least some kind of pseudo-philosophical underpinning. (As opposed to, say, religion in BSG, which doesn't seem to be about anything more than prophesies and the question of how many gods there are.) I'm thinking in particular of Foundationism, and Season 5 cult of personality that grew around G'Kar. In both cases, there are actually some *ideas* put forward about what the religion meant (though some of those ideas are arguably kind of paper thin, there's at least the *pretense* of religions being about something).
- The show as the vision of one man, JMS, who wrote 80 some percent of all the scripts.
- You write "extended story arcs encompassing long periods of time". But that barely scratches the surface of what makes B5's story arc unique. The fact that the entire story was planned out in some detail beforehand really shows. I mean, it *really* shows when you compare to basically any other long running series that relies heavily on story arcs. B5 has all but ruined the rest of television for me in this regard.

In every other show, they can create the illusion that there's some coherent long running story thread running through the series, but by at least the second or third season, it starts to become painfully obvious that they're just winging it, and the end product never holds together nearly as well as B5.
- Relatedly, there's the fact that (almost) every episode has at least an A and a B story (and frequently a C story). Usually, with one storyline connected to the overarching story arc of the series. Virtually every episode contributes at least *something* to the overall story arc.
- One particular political bent to the show that doesn't get enough attention (which is kind of related to your "the advantages to community building" item): The show is fiercely anti-nationalist. There really aren't any major recurring bad guys who are just in it for themselves, except maybe Lord Refa. Every other "bad guy" does what they do primarily because they're out to protect the interests of their faction (Londo does everything for the Centauri, Bester to protect the interests of the Psi Corps, Lyta in Season 5 to protect the interests of Telepaths, etc.), or because of ideology (the Shadows and Vorlons put their own ideological dispute ahead of the interests of the younger races). It's the good guys (the "Army of Light") who transcend all of that and put the interests of the whole "community of races" first.