I'm not a MUD old-timer, having only started playing MUDs about a year ago. But I've visited a lot of MUDs in that time. And Icesus is the only MUD I feel a strong compulsion to revisit.
A lot of MUDs pride themselves on their roleplay. Some demand that you establish a character description before you can become a fully-fledged player. Myself, I don't care for that stuff.
On Icesus, you run into people who have taken their names from Infocom games and from Buffy characters. So what? You're more likely to compliment them on their choice monickers than anything else.
There's nobody ordering you to speak in Ye Olde English style, or to distinguish between your online life and your real life. Obviously, you get warned if you start talking about quest stuff on a public forum, but there's no issues with talking about this kind of stuff directly, between consenting adults.
The reason I seem to stick with Icesus is that there's plenty of stuff to entertain me. You can do the hack and slash thing, or you can just fish, or go hunting. You have maps to guide you on the website, and an automapping feature to help you get around, so working out where you are is never an issue. The world of Icesus is interesting enough. I don't have to roleplay to enjoy the experience..
One of the neat things is that you can get permanent favours, not by paying money, but by reporting typos. It rewards players for improving the game.
I'm not that fond of the the fantasy genre. I enjoy the craft aspects of Icesus (particularly the fishing) more than anything else. While the world of Icesus is obviously not realistic, it seems to have been strongly influenced by the experiences and understandings of its Finlandic founders.
I am constantly discovering new things in Icesus. I've recently discovered the joys of hunting in the tundra: in Icesus, even snow has a personality.
I must admit that I do check out other MUDs every so often, but I always come back to Icesus.