IMO here are the main differences between Franco-Belgian and American comics:
1. not a single comic (at least the ones written before the Marvel movies began dominating the culture) is about superheroes. Every protagonist is a normal person, though sometimes extremely competent. This results in much more variety in the industry. I'm not sure why they didn't even try to copy the American model if only for business reasons, but I'm thankful for it.
2. Each comic usually gets a single 50-page issue per year. (Seriously, there's a series I've been reading since I was 8, and you can marathon it in a few sittings.) I think this reduces the amount of "filler issues" or low quality churn, as the author has lots of time to get things right.
3. Most of the time the comics are written by the same author, usually the creator of the characters. They're not getting pimped out to every author out there for a run. It makes things more cohesive IMO.
My favorite series is Thorgal, by the author of XIII and Largo Winch. It's a fantasy series set at the time of the vikings, with light sci-fi and magical elements. Think of it like Conan but with a non-violent hero. Read issue 9, The Archers, for an standalone story that can give you an idea of what it's like before you commit. It's been running since the 80s, but once the author retired a decade or so ago, some younger guy took over the writing, and you can see the quality of the writing drop off, so I never kept up. But you got 30 years worth of good books, about 35 issues IIRC.