SAO had a decent concept (people trapped in an online game where in-game death means real death) completely shattered by hack-tackular execution.
It's pretty clear that Reki Kawahara never played an MMORPG before writing SAO and thus the numerous flaws in the game itself were apparent to not only MMO fans, but people with a casual understanding of them.
Of course, it doesn't help when a chunk of the series is dedicated to romance and an AI child rather than GETTING THE FUCK OUT OF THE GAME
And really, two years? It took two years and the most they got to was Floor 75 of 100, and even then they only got out because the main character found out the guy that trapped them in the game was in the group?
That's not even getting into the pandering... as people have said, Kirito is a male-power fantasy for Otakus, and every girl wants to jump his digital bones, and each girl he comes across is an anime stereotype. The shy one, the loli, the friend of the main love interest that has no chance, and then Asuna... "Hey everyone, check out this badass chick! ... Shit I don't know how to write anymore scenes of her being awesome. She's a submissive tsundere now!"
Of course, everyone that isn't Kirito is an idiot. The guild he joined that fell for the most obvious trap ever, the hatred for beta testers that makes absolutely no sense (YOU ARE LITERALLY FIGHTING FOR YOUR LIVES! PUT YOUR TRUST IN THE PEOPLE WITH EXPERIENCE YOU MORONS!)
And this is all just Part 1... Part 2 has an entirely different set of issues to deal with.
Shit, I typed way more than I intended.