So most typical campaign settings for 4E have additional options for players. They can try their hand at a new race or class, all the while every other option is available from the various Player's Handbooks. I think at times it feels tacked on, but I get what WotC is doing. They don't want to limit how people run their games, but add to it. So every campaign setting released has fit this notion of laying out the basic options and adding other things for players to tool around with.
I think with Dark Sun that might change. I'm unsure if they will limit races, but I think dropping the divine power source is in the cards. I like it. I think with Eberron and Forgotten Realms released, WotC can now get out some really wild campaign settings away from the typical fantasy norm (even if one is a little steampunk).
This leads me to what I've been doing with my own homebrew campaigns. Originally I wanted to keep everyone happy and not put any limitations on races or classes. But I found out after a while, even with rolling up new characters, having no change in the settings from campaign to campaign made things a little stale.
Having new races or classes definitely adds to a player's excitement with a new setting. A unique feel of the world helps, and having a setting with slavery, or undead commonly interacting with the living, all helps to keep the interest of the group for a bit. However, I think what really makes for an interesting setting is the limitations in choices for players.
With 3 PHB out now, I have no problem cutting out races, and have done a little too with classes. My current campaign is a bit of steampunk thrown in a typical fantasy setting. Most races are abound, but I dropped off devas and shifters, and completely threw out PHB 3 races/classes. The only addition was introducing artificers as a player class. At first I was worried about hampering my players options and I did get a little grumbling. But I think once a few choices were removed, my players were able to quickly think up some interesting characters.
I think my next campaign I might explore this further. I really think this is another neat concept of the entire role system. I think with previous editions, I would have to completely dump any idea of dropping divine magic. No clerics? No way would my group go with that. But given that you can find leader roles in other power sources, it becomes a possibility now. Each power source has a few different roles tagged with them. Want a world where nature has been tamed? Dump the primal power source (and the classes with it). Immediately you've got a campaign setting that has a different feel to it for the players by removing these classes.
I need to think about it. And of course, it largely depends on the players willing to roll with it. I'm definitely in the camp of limiting player options for new campaigns though. A quick and dirty way to give the world a different feel, and make further descriptions of the world have some resonance for the players. So have folks done something similar with their homebrew worlds?