For me, Arm's strengths lie in its roleplay environment. The game is the most intense RPI I have played (and before I settled on Arm as my home ~2009, I played a LOT of MUDs). The commands that really stand out to me that help flesh out roleplay are the 'think' and 'feel' commands, which when combined with the journalistic biography tool that allows you to write your character's story as it's occurring, make the game more immersive than just about any other game out there. The game environment is harsh and beautiful, and one of the more unique environments to play in. This is definitely not Fantasy Environment #945 or Space Environment #529. A lot of time and energy has gone into developing the world and its lore, which is as complex and full of hidden caches of amazing as you would expect from such an old game.
If you like feeling as if you're part of an interactive novel in a gritty setting, this is the game for you.
I do have to rate the community as middling. I used to stay that one of the best things about the MUD was its community but I feel that has changed over time. I love a lot of the individual community members, many of whom are very helpful and many of whom have become my friends. But there are certain players who definitely give the community a 'jaded veteran' feel and are more than ready to shout down any new ideas and changes. Or bring up their experience from fifteen years ago as a reason why X new thing can't work. Similarly, there do seem to be periods of player/staff strife. Staff get harassed by players, staff get burned out, and a certain set of players is quick to jump on anything like showing human frustration as evidence that staff is unfair and not properly servile. I have a lot more fun when I'm not actively reading the forums or trying to engage with the community. But to be fair to them, the community is a lot less toxic than it is surrounding... other games... and the forums rules tend to keep things civil.
In contrast, my experience with staff has been largely positive, though I've had my disagreements with particular staff and staff decisions in the past. I have filed staff complaints (you can file staff complaints and they don't retaliate against you, which I have found to be amazing!) and have had them handled reasonably and fairly. Staff have also been very forgiving with problem players, and there have been several periods of both game and forum amnesty. Staff are responsive, fair, and happy to work with you if you are happy to work with them. I find that approaching things with a positive attitude tends to make interactions more positive, even though sometimes it's hard to give people the benefit of the doubt over text, which lacks body language and tone. I can say that the staff are the most devoted to fairness and reasonable procedures for enforcing fairness that I have ever seen.
The game is constantly developing in terms of code. The release notes are full of positive changes. And while some of staff's changes don't seem particularly popular at the time, because what fandom likes change amirite, the majority of the time, even the changes I haven't agreed with have had a positive effect on the game overall. The only code thing I have ever wished for is a more robust crafting system, which the game has made several steps toward in recent years. The staff seems to be actively working to improve the code aspect of the game all the time.
So that's my experience! I came for the roleplay, I've stayed for the roleplay. If you want gut-wrenching intense roleplay that will make you itch to log in to see what happens next, this game is for you.