Players are often lazy. While they enjoy playing games, they might not share Game Master's enthusiasm to the campaign setting and it's world, and as getting to know it could require reading somewhere from 10 to 150 pages, they might not really know or understand many things that come up during the game.
When designing or running an adventure, you need to be careful about how you use details. While details can make a story alive, they can also make your players fall asleep.
Having heard that abilities are being rolled again in DnD Next made me think about randomness in character creation. I'll have to say that I've missed random ability generation, and I'm a bit bored of creating exactly the character I want. Rolling the scores (dropping the lowest d6) and then assigning them to abilities you want has something I just like.
I have way too many game worlds and genres on my 'to-run' list, so I have no hope of running many of them as a campaign. I would't have enough material to run a campaign for many of them anyway, but on the other hand, I wouldn't want to run them as straight one-shots either; Players often want some continuity, so a series of one-shots doesn't sound so good.
Or, would you rather play Human warlord or Lord of Raldwik, Hero of the Three Rivers Battle?