It's the brilliant fight scenes. And the fact that Jason Bourne is really kind of hot. But mostly the fights. Do I want to analyze why these movies just did "it" for me, from the beginning? No?
There's a lot of things that come together in the movie trilogy. But from what I've read on the novels - and I don't doubt they make for a good enough read - they don't seem that interesting to me. And I say this as someone who's read more than one James Bond novel. I think it's the fact that they feature the Jackal. I've always loathed spy-fiction that featured the Jackal, because they blur the line between fiction and reality. And somehow I'm concerned that if I read the novels and hated them, it would take away from my enjoyment of the movies.
(I'm not counting Lord of the Rings, because I didn't really enjoy the films, either.)
There's a detail - two, as a matter of fact - that I'd like to touch upon, though. Ass-shots of the asset that wasn't Desh. There were two ass-shots of him climbing a ladder that sexualized him in a way that women are usually sexualized in Hollywood action films. It reminded me of the comic-covers meant to subvert the sexualization of heroines by presenting heroes in similar poses to them. That made me realize that I also love these films because they have strong female characters (despite the fact that it's really Jason Bourne's story that's central to them) that aren't reduced to sexual objects. See? It can be done. Marie is also a good example of how a female character can be killed without fridging them. It wasn't cheap angst, she mattered. She had agency. She taught Jason more when she was alive than by her death, which really taught him nothing.
So, yes. <3