Sword Art Online is...well I really don't know.
I put 15+ hours into it and it felt like nothing happened. Characters kept getting introduced, there were all these scenes, and the plot was just ?????????? The bulk of my play-time was spent in an apparently entirely optional area. I didn't even discover the area I was supposed to go to until the 13 hour mark.
The combat is interesting, sort-of action-MMORPG. I say sort-of because it's not really free. You can't just run around and dodge everything, you have to properly use your skills and evade at the right moments.
The cast is just the worst. So many girls and all they do is fawn over the main character. It's to the point where I'd get disgusted. Sometimes I just want to be left alone so I can figure out what I'm supposed to do, but dozens of things are happening and all the waifus (and an actual virtual wife?? and a kid?!?!?!?) are hassling me.
The story of the game is...not the strongest part of the package; the large world and interesting combat mechanics are why many like myself really enjoyed the game

Regarding the story, look at the following:
-Takes place in a VR MMO, but the characters cannot log out due to that feature being locked out by the creator.
-Leads to everyone going into a panic, but coming to the realization that they need to beat the game to be free (which is 100 Floors of dungeons).
-Main character Kirtio was a beta tester for the game before its launch, so he is skilled with its systems
-He meets his harl....I mean, cast of female friends across the anime
-Gets close to a female player named Asuna and they become 'married' in-game.
-Their 'daughter' is a AI program that they encountered in one of the anime's episodes, who they took in as their 'real' kid
-Final battle in the anime was in Floor 75 and in the anime, after the fight with the creator of the game, it ended, with everyone going free
-This game (Hallow Fragment) takes place in an different reality, where the game DOESN'T end, it just goes on and glitches out
-The cast of the anime have to deal with this and continue through the floors until they beat the final boss on Floor 100
-Character introductions are very clouded, so I will explain them here: Sinon was bought to the world due to a glitch during VR therapy. Leafa (Sugu) was brought to the game on her own will; used a friends NerveGear (the things they use to play SAO) and loged in to see her brother/cousin. Strea is a 'new' player in the game world but is a AI program similar to Kirtio's daughter; originally a mental-health montaring program in the game but was brought out with the other players after the 'glitch'. Philla is a player affected by the 'glitch', who after 'killing' another player, is locked in the Hallow Area (the first playable area in the game you see) and you work with her to fine a way out for her
-Everyone knows Kirto is married to Asuna but they don't care because they all want him :l. Hey, the game creators couldn't fix everything from the anime XD.
Hope this clears things up for you as outside of the story, the core game is very solid.