I didnt start wathcing Sword Art Online until there were already 19 episodes, and I wound up watching them all in one night! I am an MMO player, but I dont think you have to be one to enjoy this anime. (but you will probably like it more!)
Dont read on if you dont want any spoilers!
First off, the romance is not your typical anime romance that just happens, or never makes progress. This one actually develops, and it was suprising to me how it developed.
The concept has been done already (mmo world having real life consequences) but there is still a lot of variety available within the concept, so to say 'its been done in '.hack' or 'accell world' would be inaccurate.
Characters are built well, there are of course the characters you are ment to hate, and those your ment to love. But there are also inbetween characters you can like or dislike without being swayed by the writers. Its easy to figure out who the main characters are, but its not so easy to figure out who the supporting characters will be playing larger roles. This is good in my opinion, because it gives away to much about the future of anime when you know who will impact the future episodes.
The animation is excellent, not so much in the artistic ability/style, but also in the fight/heavy action scenes.
I have already watched episode 1-19 twice (had to review for episode 20) and I still enjoyed them and noticed things I did not see before. The replay value is good.