Hi
... mpeg or divx for movie playback (Warcraft III uses another codec).
That's actually not true. The movie files have been hacked to supply a false codec id (so the game would pull in a proper decoding backend) but it's actually fully DivX 5.0 compatible.
Warcraft had suplied a binary decoding-only library crafted by the people at DivX.com just for the purpose of the game.