Faction quests, MQs and the rest. I think Brian Chapin wrote the Oblivion Mages' Guild, although I'm not so sure, the UESP only said that he was "in charge of it". Emil Pagliarulo wrote and designed both DB questlines, at least that's what I heard. I don't know about the others, I only know Emil is senior writer in Skyrim so he could have written the MQ, but I heard he spent a huge chunk of time with the dragon language, combined with the knowledge that he also probably wrote and designed the DB, that tells me that maybe he wasn't that involved in the actual writing of the main quests. So who knows some more specifc information?

I have no idea but would love to know as well.