I've been re-watching Fullmetal Alchemist (both) lately, and I just realized something pretty cool about it.
Most manga/anime I read/watch has specific arcs. The adventure's continuous, but split up into separate segments. Hunter x Hunter, for example, has pretty clear sections (Exam arc, Yorknew, Greed Island, Chimera), and while they all flow together pretty nicely, each arc has its own set of problems and characters and whatnot. Gintama's also a great example but I think it works a bit differently.
Maybe I just haven't been observant enough with FMA, but FMA feels like it just tells one continuous story. Of course there are new developments and new characters and whatnot, but it never seems like it has any clearly defined arcs. The whole story just meshes together so well, like one thing just leads to the next, and not all the problems are solved when the main characters move on from the area.
I also see this flow in other series, like Code Geass and Attack on Titan (well I do think there's a bit of a split between the earlier and later chapters but in themselves they flow together really well and enough stuff happens in them for them to not just be separate arcs).
Anyway, I just think it's cool when manga tells one continuous story.