Warcraft III terrain system is terrible. Due to its use of tile sheets it cannot even use basic mipmapping with the terrain, resulting in aliasing if a decent tile resolution is used.
Oh and the visible seams has nothing to do with the terrain engine. That is just a Blizzard art bug since the tiles were not constructed with proper wrapping boarders. It is completely possible for content authors to create seamless Warcraft III tiles with the current Warcraft III terrain system. Doing this makes it look a whole lot better and less dated, despite still not using mipmapping.