Islands of Myth.
What got me hooked is really the essence of the game itself. A multiplayer fantasy RPG with a global player base. It's a really cool concept that I've enjoyed in many iterations. EQ, WoW, GW2 etc.
I just never really stopped playing this one. One of the things I love about MUDs is that it's so easy to just have it running in another window. Killing a monster every 4 minutes during the work day. Chatting with folks and organizing groups while literally playing other MMOs.
I don't think that I could play WoW on one screen and GW2 on another with any efficacy and I almost certainly wouldn't enjoy it.
I've also really enjoyed the classes the game offers, the ability to multi class, and the reincarnation system which is similar to modern "prestige" systems. Except I can keep most of my earned exp while choosing an entirely different race and class combination.
MUD communities tend to be pretty great and this one has been no exception.
In regards to the screenshot "Mud age" is the same thing as /played or /age in MMOs. The amount of time spent logged in to the game.
While they're often full of older and or OP players, most of them allow for secondary accounts / characters. So you have folks playing on new characters as well.
Islands of Myth has a feature that lets you split your character into multiple "bodies" which you can switch between (with a cool down). I can earn 100 million experience points, spend them on levels and spells, becoming a strong healer. I may decide I want a change of pace and split that 100 million exp 80% 20%. Now I have a less strong healer with 80 million exp to spend on levels and spells, as well as a bard with 20 million exp to spend. You can always combine them back into one body later.
This is not an entirely unique setup, so I think you'd find most places with a nice mix of player "sizes".
Would I recommend IoM for a new MUDder? Yes. It was the first MUD for a lot of folks, so something is being done right. That said, I'm sure there are MUDs with smoother new player experiences. For example IoM requires players choose, install, and configure a MUD client, while others offer a web based client or custom app.