1: friends that play - this makes the game fun
2: I love the graphics/style of the game
3: It's hard - it's terribly difficult (if not impossible) to solo to max level, making parties more essential, meaning you need to know how to play well, or you're going to be out a party.
4: sense of accomplishment - last time I played, I was level 12 before my trial ran out (had no money to sub) and I had just gotten my Scale Mail set, and even at such a low level, it felt like I had actually -done- something. Like I actually earned the damn full set of armor.