I'm talking more significant, multi-stage quests, like major enough that you actually get achievements for beating them.

I've done the main story,

Dawnguard

Dragonborn DLC

Thieves' Guild

The Dark Brotherhood

The College of Winterhold

The Companions

I haven't done anything for the Stormcloaks or the Legion, but I really don't have much interest in doing so. (Am I missing out on a lot, though?)

Any other major questlines I missed?

have you checked the wiki, alot of side quests and stuff are listed there. do the various cities, and the wilderness chains too

Other than the CW, you've pretty much done the big things.

There is a side quest that goes on for ages. It's called Arniels *something*

I really enjoyed the quest line for the Legion. I reccomed it. Will try the stormcloaks on my next play through.