RPIs are the best kind of muds. I used to dig the Iron Realms games back when... well, when I first moved from hack-and-slash MUDs like Vampire Wars to try roleplaying enforced ones. They were pretty fun, but basically they're all about grinding and setting up countless triggers in your MUD client to have any chance in PVP.
With an RPI you have actual risk and reward (thanks to permadeath), mostly excellent, easy-to-use commands and such, and of course some truly brilliant storylines when good players contribute. Characters wind up seeming very real and it's usually incredibly easy to find yourself truly in the thick of plotlines, or spark them yourself.
Sadly, I don't think there are really any active MUDs around any more apart from Armageddon, which I've never been able to get into for a variety of reasons. It's annoying, really- I've always enjoyed reading the forums and the occasional stories and anecdotes that bubble to the surface there, but I've never had a serious character myself.
The whole "Find out IC" stonewall everyone throws up probably doesn't help matters.
Shadows of Isildur was dope back in the day even by the time I finally discovered RPIs. Before that the first one I played was the alpha of Atonement, which blew me away-- but it still had nothing on the fun I had with SoI, especially after I started playing orcs. Oh man, those were the days. I remember so many all-night roleplaying sessions. That game was basically my life for a long time, haha. Somehow it inspired me to lose twenty kilos along the way, too!
If I had the coding knowledge (and wasn't a lazy fuck) I would love to use the old Atonement/Parallel codebase to create an RPI in some kind of Western setting. Seems like a goldmine of untapped potential, there. Those game's engine would be perfect for it in so many ways... hunting and scraping through dangerous badlands to survive, all that frontier living shit, crafting and of course the brutally unforgiving gunfights... that'd be amazing.
Sorry for the ramble. I just miss mudding, that's all.
The only roleplaying experiences that can hope to compete with an RPI are things like SS13, forum games and... well, I guess certain tabletop games can match RPIs for the full package, with great roleplay potential and fun mechanics to match. But for that you need people, and then seriously roleplaying with people you know can be awkward!