Armageddon is an incredibly intensive and above all realistic role-playing experience, and also has a brilliantly designed player character system.
Your character's (whatever he/she is) attributes, skills, statistics, etc., are described with numbers in only a very few cases. You only know the precise number of your hit points, stun points, mana, and movement numbers, and these remain more or less the same (with some fluctuation according to age of the character). Although it sounds restrictive, this helps the atmosphere of the game and actually simplifies your life, because, although Armageddon has a lot of hard code, knowing your precise strength (for example) has no use here.
Strength, agility, constitution, and wisdom are described according to your character's racial average (for example, "very good", or "exceptional"), and these will probably never change except for normal ageing (for example, wisdom keeps increasing indefinitely, but strength peaks in maturity). All derivatives of these attributes (for example, saving throws or damage bonus) are hidden from the player. This doesn't mean you don't get the information, merely that you have to think about it based on the world of Armageddon (for example, a dwarf with "exceptional" consititution is probably immune to poison, and Half-Giants usually are so strong that the damage they do is incredible).
Races on Armageddon are not a matter of racial modifiers (such as +2 str, -2 agi, -1wis). None of that nonsense here: races are very distinct from one another, and extremely different in terms of physique, character, appearance, and abilities. Desert elves pride on their ability to run extremely long distances that other races can only match on a mount. Muls are expensive cross-breeds of dwarves and humans, bred for their physical power, and are almost always slaves or gladiators.
There are no levels on Armageddon. Your character's hit points will not change substantially through his life. Everything on Armageddon is built realistically (there are no mobs with 15000 hit points!), said that there are some _very_ scary inhabitants, ranging from the gigantic to the tiny, and many of these are very deadly (all of them are richly detailed).
The elaborate branching skill system is based on classes and subclasses; to advance a skill one has to practice it, and I don't mean going to the Guildmaster and using your practice points, but actually PRACTICE the skill in question. The only way a warrior is going to get better at fighting is if he fights (and survives) enough times to learn from his mistakes and build up his abilities. The same goes for a burglar, assassin, thief, ranger, merchant, templar or whatever, as well as for all the subclasses. This helps to build your character into a full role-playing experience.
There are no distracting out of character channels. Once you log on to Armageddon you are in another world entirely, where everyone and everything is in-character all the time. Coupled with some truly well-written and very imaginative descriptions of the harsh desertified world of Zalanthas, this makes for a very engaging time.
Armageddon was inspired by the series Dune and the Dark Sun books and RPG, and the world they crafted is one where you can almost taste the gritty sand blowing in the wind as you sit at your computer. I have never encountered anything like it in a decade of MUDding.
Equipment and clothing are also very realistic, and 99.5% of it is more or less ordinary stuff, with the remaining 0.5% being made from prized materials, very skillfully, and perhaps even using magick. Most people have never even seen a steel dagger, let alone a steel sword, because metal is even rarer than water. Obsidian, bone, and wood are some of the materials for weapons. Some are crafted well and some are shoddy (the item's description provides details--again, no numbers, just a description), but everything is very realistic.
The combat system is excellent but I won't describe it here. Suffice it to say that it satisfies the most rigorous tabletop RPG requirements of systems like AD&D. It is efficient, realistic, and exciting (you die, and that's it!). The magick system is likewise very engaging but it's recommended for expert players (magick is rare and shrouded in mystery, and the average reaction to it is automatic distrust).
Finally, this game is free. Considering the sheer amount of work (12 years!) that went into it, and the imaginative and efficient administrators (immortals is a bit of a misleading term here), I am amazed--this game is a work of love on the part of several talented and imaginative people.