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I mean, they're both popular MMO series. Though SAO was popular because of its anime, while .Hack was a joint effort between the games and the anime (the latter serving as prequels to the games in its two main entries). But .Hack isn't a VRMMO, and it puts more emphasis on character development and story than the fighting (though SIGN and G.U. did a good job with world building).
The games are fantastic, and G.U. is one of my all time faves. .Hack//Sign is a really good anime with Roots being...eh. I hear Quantum's good, too.
Ugh, now I'm sad <_<. I love .Hack, but whenever I think about it I can't help but feel that, as fans, we've been forgotten. You can't even find the PS2 titles cheap, and the games haven't been rereleased or HD remastered :/