I've thought to myself for a long time,
"How can AoT, a game so undeveloped and with so little gameplay, be so addicting?".
Here are reasons I can think of -
Customization - You can pretty much wear customized clothing, change your hair colour - Standing out
FPSishness with RPGness, if you know what I mean.
Beautiful scenery - If it was just a boring, flat landscape with nothing but grass and a few trees, I doubt we'd play it as much, if at all.
There are other reasons, so come on... Post them.