I think "rocks" do a lot to create a sense of environment . . . but they need a lot more than just rocks in order to create a distinct environment. Trees, water, architecture . . . they need to be doing more. Star Wars looks like it has a lot going on besides the rocks. Avatar has its bioluminescence, but it remains to be seen if that will be enough. The model they're showing off this weekend is inspiring confidence within me. Cars Land is in at least some ways defined by the sheer amount of rock. It's really SO much; enough to an impression so striking it's almost absurd. You're right, though, it doesn't work if every addition is making that same "statement".
There's so much more to Design than just rocks.