Howdy neighbour, welcome to Cyrodiil :wave: We haz http://www.uesp.net/w/images/images.new/c/c4/Fishystick.jpg too, but I think they taste better

I think the MQ is a bit shorter than that of Morrowind's...there's a lot of running around and fetching stuff again, but overall it seemed more concise to me, with the bulk of excessive time coming from the optional side trips. Shivering Isles is the only expansion this time, and just like Bloodmoon, its questline is separate and unrelated to the main MQ.
One of the biggest differences you'll be seeing right away is that the world levels with you. No more "can't go in there yet," because you'll nearly always be able to take on whatever's hiding in the shadows. Quite frankly, I died frequently around Seyda Neen, and even around Mournhold and Solstheim, but in Oblivion, dying was extremely rare.
Roleplaying is TOTALLY possible here too

I brought an Orc over from Morrowind and continued his adventures, and had a great time doing it. Many here roleplay and have favourite characters -- mine is a tie between the first time I plopped myself into one of these games, and using a mod to make one of the bad guys playable and watching him become a holy defender of the Empire.
If you ever used the Construction Set, the OB version looks and works the same, but takes a lot of getting use to.
And yep, casters are totally doable. I'd never used magic in Morrowind, but in Oblivion my favourite characters end up being casters. If you feel like giving magic a try, buy some spells and play with the different schools, and see what they all do