I just don't get Reki Kawahara's obsession with MMOs. Say there are two anime series about exploring the galaxy, fighting aliens, etc. but one is in an MMO. What reason do I have to choose that one? Unless you work in some ridiculous circumstance where you die in real life the MMO version is at a disadvantage as there is no tension. During the second half of SAO why would I care about any of the fights prior to the absolute final one? I wouldn't because it's Kirito, but anyway, what does him loosing some gold or experience have to do with the grand scheme of things?
The only thing I see the MMO version having over the non-MMO is that it appeals to people who can't socialize outside of MMOs themselves.
note: I haven't watched Accel World, I'm just commenting on the MMO bit.
MMOs are closer to your audience.
If your anime is set out in the galaxy fighting aliens, even if viewers aspire to be a part of it, they know that they will probably never be able to go out into the galaxy and fight aliens. But if it's in a VR MMO, that's something that maybe could exist. They could live in that world too! It's the same kind of fascination that exists for settings like Harry Potter or even the Type-Moon verse, where the magic exists just around the corner from our own reality.
And in the end, a setting like Aincrad holds the same fascination as any other popular fantasy setting. It's a world brimming with undiscovered secrets and hidden possibilities, where, no matter where you come from, you can find something new and incredible, and you can experience something special and unique.
I know some people find the distance between the real world and the virtual world to be too much of a problem to overcome (it's all Firehawk seems to care about), but I'm sure fans of the series don't really care. Wherever the fantasy occurs, the sense of possibility and discovery still exists.
