Warcraft 3 has a steep learning curve, like RTS games do in general. One mistake can cost you your hero, which can cost you your hero if the death is untimely. (I guess DotA derivatives are more forgiving in that sense.) But there aren't that many units and heroes to learn, so at least after some time you won't be taken completely aback when your hero gets ensnared, critical striked (struck?) hexed and then critical striked again. I mean, it will happen, but the possibility would have already crossed your mind.

There is a ton of finesse and micro to playing a game like WC3, but learning to play the game isn't that hard; there isn't terribly much that you need to memorize, and then you can concentrate on strategy and mechanics. DotA (derivatives), on the other hand, is built in such a way as to be absolutely relentless towards anyone who isn't already experienced.

WC3 is hard to learn, but not unreasonably so for an RTS. It also has a high skill ceiling. DotA (derivatives) are really hard to learn (I won't comment on the skill ceiling, since I don't know). Yet, people - casual players included - have embraced DotA over the humble original game. I just don't understand the attraction, particularly when it comes to the more casual players.