I have seen private servers with custom models for
WoW, but I have never seen custom terrain created or zones/instances. When I have asked, people always tell me it is much too complicated to do terrain. Any ideas to why this is? Just curious. I remember once I jumped off a map in real WoW server and my char completely froze and was completely unable to do anything so I would surmise that the code of how the objects will behave is somehow tied to the terrain?

as for creating your own terrain, thats gets really deep into the mechanics of the game. You would have to have the tools that the artists/programmers have at blizzard. I'm sure they have to make a level, add a bunch of crazy collision algorithms (prolly written in c++), then compile it into the game, make sure every thing runs right... lots of work. [At least what I think,] you would have to have the progs and code from blizzard to do it ^^ hope that helps

You know creating a terrain from heightmap is not hard. Its
actually easy once you understand the concept.