As has been said in the comments, the point is largely the art; functionally the basic lands are always the same and always legal. Sets are designed with a coherent theme and world in mind, and the lands should fit with that as much as the other cards should.
You can see a fairly convincing demonstration of this in view of lands in Innistrad and Avacyn Restored. They're all the same locations, but while in Innistrad they were dark, cold, and barren, in Avacyn Restored they're brighter and warmer, often with more life. It's well worth looking at the link for all the full-sized art, but for example, here's one of the mountains:
Alternatively, you can just go to Gatherer and click through the various sets to see the huge variety in art! Here they are: Plains, Island, Swamp, Mountain, and Forest.