Indeed, the main story is quite short, but there are many hubs for side quests. Overall, there's a lot more new content of all types (quests, exploration, items, mechanics) in Dragonborn than there was in Dawnguard, it just lacks the epic-length main storyline. Also, they added most of Morrowind in more detail so you can see/walk on it, though I assume it's still an out-of-bounds area; there are no cities, vegetation, etc, but you no longer fall though it...

I particularly liked Apocrapha; there were times when I felt like it really was pulling me toward insanity. It's unfortunate that you can't reset certain parts of it; I blitzed through most of it for the main quest and missed some spots. Also, I later decided I wanted to build up my library (definately the most book-loot-rich "dungeon" in any Elder Scrolls game I've played), but some earlier areas were inaccessable.

As for the landmass itself, it mostly feels different, though it's been a long time since I've played Bloodmoon. Obviously, hundreds of years change things, though...

Yeah I'm taking my time with it; playing it here and there.

I am enjoying it though. Feels like an extension of the game rather than separate Add-on Content, which is why I don't mind that the main questline is so short, given it's like another one of Skyrim's sidequests.
I wonder if my bias towards any add on content has swayed my opinion of it. I've essentially exhausted Skyrim of new things to do and explore, so anything new and unexplored is appealing to me.