I think the atmosphere and immersion is what makes Morrowind so enjoyable and the pace seems just right, I can spend a long time just randomly running around the map finding things to do when I want a break from questing and never get bored. Oblivion I enjoyed a fair bit as well, but I don't know what it is with Skyrim I just can't play it for more than 15-20 minute bursts and it doesn't really feel fun for that small amount of time. Not sure why I can't get in to Skyrim, but something I can't put my finger just feels missing.