So rather than try and come up with my own original description, I'll just steal this one from dothackers.net:
"The World" once again flourishes, with over 10 million players playing "The World R:X", set in 2022. In a special Christmas event, three players set out to conquer an "unbeatable" quest: "The One Sin." .hack//Quantum tells the story of a party of adventurers: Tobias, Mary, and Sakuya, with familiar looking Avatars of past players. The OAV will be split up into three episodes.
Before I go on, here's a helpful video or two to give you an idea of what the heck this looks like.
Finished watching them? Yes? No? GREAT! Let's move on!
So characters! We have Sakuya (the girl with the long twin tails and orange outfit), Mary (long pink hair, little bikini top), Tobias (silver haired bishounen one), Hermit (the cutest little cat thing EVER), Shamrock (blue haired goth loli) and Smith (big golden armored man). These are just as far as we know right now, anyway. Also, the previously posted link has some info on the main trio's offline selves as well, which you see a bit of in the second video.
As previously mentioned, Quantum is going to be a three-part OVA series; apparently each episode is about 35 minutes. The first episode will be premiering in Japan later this month on the 27th; the first BD/DVD release is set for late January.
Other cool information can be found here, assuming you can navigate around Japanese: http://www.hack.channel.or.jp/quantum/
I'm so so so sooo excited for this. *v* It just looks like something that can really appeal to a larger audience, not just the .hack fanatics. Quantum can still prove me wrong on that, of course, but either way it's something I'm highly anticipating. And hopefully I've interested some of y'all now! 8D