The constant updates, additions and wide variety of content to the game, making it so there is always something new to try when I get bored of hunting.
The very slow skill gaining system that really makes getting to something like serendipity feel like an real achievement.
The way I'm still such a newbie after two and a half years.
The way I can vary my deposits to match what free cash I have, not be stuck to a set charge each month. Which is especially good when I can still play in a month where I don't feel I can spare the cash to deposit.
Being able to mindlessly blast arbitrary mobs away to relieve the stress after a bad day.
The occasional high of getting a nice globie when you don't expect it.
Oh and ofcourse, the community, my soc mates, the forum, and generally nice people that I meet out and around the place.