It's comics in general. Even graphic novels...I've had trouble really grasping what it is to them that I can't get out of a book or a movie.
For me, the magic of comics comes from nostalgia. My brothers and I were
not bought comics as kids, although we did read the Sunday strips in the newspaper every week - including a wonderful, now-rare, newspaper introduction to the character of Batgirl, which ran during the weeks and months before Batgirl turned up on the third season of the wonderful "Batman" TV series in the 60s. And, if we were sick, Mum would buy us a few British comic books, "Beano", "Bunty" and "Sparky". And a "Phantom" comic was traditional in a Royal Easter Show showbag!
Then, perhaps every 18 months or so, we'd receive a brown paper grocery bag of comics, passed on from a cousin we hardly ever got to see. In that bag would be Disney and Harvey comics, plus the odd "Archie" or DC superhero comic. Once there was even... a "Teen Titans" comic! We would devour the whole pile over a week or so, often sitting in the tent belonging to the girls next door.
It was only many years later, when I'd see an interesting title on a newsstand, that I realised how expensive a single comic was, especially for the brief time involved in reading it. However, a beautifully drawn, well-written comic does withstand multiple readings, and even multiple casual browsings. So when I found Star Trek fandom in 1979, I rediscovered a few Gold Key ST in second hand shops... and then Marvel's new post-TMP line. As bad as some of those were, the nostalgia was also strong. Different to watching the ST movie at the cinema, different to episodes on TV, and different to reading the Bantam novels, but better than Gold Key.
About the same time, I began to follow "Titans", again remembering that lone TT issue from my childhood, the coming of Nightwing, and then "Fantastic Four", while researching their uniforms for a convention.
So, what's special about Star Trek comics? I dunno. They just are.
What I loved about the "Watchmen" movie two weeks ago: even though I barely new the original mini-series, and read the first 4 issues before seeing the film, it captured the nostalgia for days gone by that, for me, is intrinsically entwined with comics.