In my personal opinion I agree with the people who say Sword Art Online was over-rated. I enjoyed the series itself and it was good but it was nothing fantastic and it certainly wasn't near a masterpiece. Broken down the series is nothing more than 'beat the piss out of everything' action with a rushed romance 2/3 of the way through the first season. Personally I feel that Log Horizon took the genre and pulled it off much better with more depth and explanation, also isn't so rushed, and the world is just much more fleshed out and explored.
As for under rated anime, I definitely go with anime such as Chihayafuru. I have brought up the title many times to many anime fan friends who have then turned around and said they've never heard of it. If you haven't, it's a definite must watch, I never thought I would fall in love with an anime about Karuta, but its so damn funny and, at times serious and keeps you on edge. Which seems weird for a game about the 100 poets. Chihaya is probably one of my favorite female characters ever, she's simply awesome and if she existed in real life Id marry her in 2 seconds flat, shes funny as hell.