Skyrim was my first Elder Scrolls game, and I also felt that the main campaign is pretty lackluster. It's extremely straightforward, with little surprises (really only Paarthurnax comes to mind) or noteworthy events compared to even the sidequests in the game.
I've heard that the different guild quests are shorter compared to Oblivion... is that true?
I think they are probably a bit shorter. I just generally felt that each guild had less missions before you reached the top. The Mages Guild seemed especially short and ended rather abruptly. The thieves guild takes a while in Skyrim but that's because you need to do lots of the minor missions before you can be guild leader. The actual questline is relatively short.
Having said that, they did also have the StormCloak vs Imperials questline which Oblivion didn't have and I do seem to get more minor/random quests due to the new quest system they have in place.