1.) I haven’t been to the LA Comic-Con, so I can’t comment on that. I can speak about the NY comic con from the early 1970s – I went to those.
2.) It was a chance to see and speak with a lot of the creative folk responsible for the comics you like (and don’t like) (I got to talk with Jack Kirby! Woo!). There were panels announcing upcoming comics, films, and TV shows (especially comic-related). There are also panels discussing aspects of comics, comics, history, and the like.
3.) You can buy (or at least look at) old, obscure, and rare comics. See a display of porno “eight pages”. See C.C. Beck’s (Captain Marvel) illustrations for The Hobbit
4.)From what I’ve seen, the LA Con is more like a science fiction convention, in that it doesn’t concentrate only on comics, but covers all sorts of other things of pop culture interest. The LA Comic Con has become the place where announcements about genre movies and TV shows get made, cast panels appear, and trailers and pilot episodes get shown.
5.) As I said, I haven’t been to the LA Comic Con, but if it’s like a SF Con , there are in my experience two types – Fan-Run and Studio-Run. Fan-Run cons are a lot of fun. There’s a lot of participation, a lot of panels that go into all areas, there are demos of martial arts, magic shows, gaming, Live Action gaming, theater, movies, an art show, readings, costume shows/competition,and an active Huckster Culture where you can buy books, artwork, replicas, movies, and unexpected things like buttons, spices, clothing, chain mail, mead, and the like.
6.)I’ve been to one studio-run convention, a Star Trek Con, and it was like the worst of the ideas even sven lists in the OP – crowds, long lines, no panels, no movies (except packaged clips), nothing to buy but studio-approved stuff from a handful of dealers, little in the way of food (overpriced and fatty, natch), no readings, a miserable little costume competition, and a lot of repetitivebess. It was an attempt to funnel fans into a handful of pre-packaged events (Come see person X speak on stage! No touching!), see teasers for immediately upcoming stuff, and buy merchandise. We left early, without even seeing Jimmy Doohan.