Warcraft Online sounded like it was going to be good, too bad it turned out you couldn't play any of the best races. The idea of playing evil looks more appealing to me, so it's back in my considered games list. However, on the case of clones;
Similarities;
All the same races, with a similar lore involved. Chaos / Burning Legion. Undead armies, etc etc. There are alot of similarities, and the game also looks very similar.
Differences;
Orcs in Warcraft aren't actually stupid as most depictions suggest. Except for Peons
Not too many differences I can think of here, except for some minor lore differences.
HOWEVER.
If anything copies anything, Warcraft copies Warhammer, because Warhammer came first. But it doesn't matter, 'cause they both copy Lord of the Rings. :P