That is a great question. I feel like you are the main demographic SE should be aiming at.
I really wish I could answer it better, but to be honest, I don't have much experience with the other games to give you a good answer. I wasn't really too fond of Guild Wars 2 or TERA. GW2 entertained me for about 3 days, before I got bored of heart quests, and TERA for 2 weeks because all the people I was playing with stopped playing...

Which basically leads me to my point: Community.
I know a number of people will disagree with me, but I have found this game's community is far tightly knit than any other recent game's community. I look at my server Durandal, I always see people doing stuff together. Regardless of linkshell allegiance, I've noticed there is a wide network of people helping each other out, just hanging around in town cracking jokes in /say, or doing Moogles for the 50000th time. Maybe its nostalgia goggles, but for 2 years, I've basically watched my server grow up like some crazy dysfunctional family, and I've enjoyed every second of it.

So yeah. That's basically why I play this piece of crap game. Hopefully you can relate.