That's one thing about these world maps.... Ok, so, Oblivion's world map was HUGE, but.. was it really different? There were a few landmarks here and there, but the map was just a repetition of itself over and over and over again.
Thinking about it, that's probably why I got tired of it so fast. The missions were always the same, the mechanics, all pretty much the same, you have a huge list of spells but you don't really feel the difference when using them, etc. In concept, it is amazing, and something I would really, really enjoy playing, but when you really get down to it, it's "a lot of just the same" and very disappointing after your first 20 hours or so. At least that was my case.
And, I guess that comes down to what personally pleases you. Mechanics in a game could be the same but it depends if you like them or not, if they'll be tedious or fun. For example, I found Assassins Creed too repetitive, but I found Prince of Persia (2008), the mechanic of finding the crystals, or coins, or whatever they were, really awesome. I also loved Batman's riddles for example (That game was just too awesome).
Also, one thing that really KILLED it for me in Oblivion was the fact that I didn't have to pay attention. Everything was put in the map for me, so I would just skip the conversations and go to where the red mark was. That certainly didn't help. When I try to remember games where I truly enjoyed a vast world, I can think of stuff like Might & Magic. However, I had to write things down in that game, figure where places where, what they looked like, etc, if I wanted to get to the heart of my sidequests. With Oblivion's point of the map, I didn't need to pay attention to anything. Open the map and it's right there, laid for you. So, that honestly also killed it for me.