Morrowind is a great game but it does not hold my attention for long. I have a game going that I have been playing on and off for over a year. There are a lot of slow moments involving travelling and a lot of stuff is repitious. But the main reason it does not hold my attention is that it is just too easy. I try everything possible to make it harder, such as not training skills and not stealing, but it just gets too easy. MM games never seem easy to me. I think it is because the games do not scale opponents to your level, so if you take a wrong turn you can easily get in a sticky situation. I also like how the world gradually opens up to you, and all the regions are different. These elements just make you want to play a bit longer to see who lurks around the next corner.
For some reason I never considered the MM games to be the best RPGs I've played, even though they are probably the most addictive. I think its because they are a little rough around the edges - unpolished. But from a shear enjoyment point of view they are hard to beat.