Of the vast number of reasons why Melee is legendary, here are some good ones:
The learning curve between never having played and winning in the competitive scene rivals any other game in history. To be quite honest, denial of such is not really something that can be argued. I don't have the problem because I own and have played both games very much (respecting HL2's quality as well), but argument against Melee can only be blamed by ignorance of its depth. The game is a challenge, there is never boredom in Melee.
Secondly, if you've bothered to watch some of the links people have posted, you'd know what a social game this is. Sure, WoW is based on interaction but only via an unengaging anonimity. Melee is a game that births communities and friendships. Melee can be more than a game. It's not just a gamer's game that one can sit by oneself with at home. HL2 has it's limit. Melee is a game of the people.
HL2 is an experience one can forget, Melee STILL lives on with no end in sight. The game REALLY MATTERS to people, that's why it is the best.