I just recently discovered the MMO Roundtable site, and on their forums saw a thread wherein posters listed their top 10 requirements for the ultimate MMO. Reading through the posts was interesting in of itself. Seeing what sort of things people focus on is very informative, and you can often get an idea of what MMOs they have experience with as well.
I was going to wait until I could cross-post this on their forum, but they don't seem to be in an hurry to activate my username to let me post (been waiting a week and half now), so I'm just going to state my ideas here and they lose out!
For many years I've shuffled numerous MMO system ideas through my head and on various scraps of paper. About a year ago, I decided to try and compile then all into one notebook, and then from there see how/if they could be integrated together or made to work as standalone concepts. This progress on this project varies as time goes by, but, at one point, I did sit down and make a list of what I called Core Concepts. Now I present them to you, with a bit of extrapolation on each item.
1) Dynamic world - NPCs, Mobs, towns etc. Very little is static. All mobs have desires, needs, wants that affect what they do within the world. Players killing them repeatedly? Well then maybe they'll move to a new location, farther from town and the ravages of a bloodthirsty populace. NPCs dont just buy up every little trinket that an adventurer shows to them. They only want things that they are interested in, or that they can resell if they're of that sort of mindset. NPCs can go broke, close up shop, switch professions, get killed, open up new stores, move to other cities, become player-followers, give birth and die. Resources can be used up or severely diminshed. Localized resources can be effected by local conditions. Towns grow and shrink based on player activity, economic viability and external threats. The more static a world is, the more quickly it becomes boring and overly predictable.
2) Component-based crafting systems - Every crafted item is created from component parts. Quality of component parts determines quality of whole. Magically enhanced components can be used to create better items (like the socketing system in Diablo, but on a much grander scale). Items can often be broken down to salvage some or all of the component parts. Spell-crafting also falls into a similar system. Spells are composed of component parts, which can be used to alter the characteristics of that spell (similar to Saga of Ryzom, but with even more options). Players can find unique ways to combine spell components to create their own individualized spell system. Certain components may actually have a negative effect when combined with other components. Parts create the whole.
3) Players can affect the world - This kind of goes along with #1, but is more specific. The world isn't just dynamic in of itself. I would instead use the terms flexible and malleable. Like a combination of Play-doh and Stretch Armstrong! Town and building creation and destruction. Enviroment and landscape alteration. Political ramifications for player actions. Put opportunities for change in the hands of the
4) No Junk System - Every item has some use in the game. If it cant be used directly, it can be converted or processed into something useful. This carries over to NPCs and Mobs. All NPCs and Mobs have some purpose. Even if they don't have a specific programmed purpose, by giving them driving factors that determine their behaviour (see #1), they create their own purpose through their actions and the effects those actions have on the world. Structured and purposeful design.
5) Minimal statistical info given to players - Number crunching is not encouraged or supported. Certainly, players have the uncanny ability to backwards research all of your formulas, but I dont want that to be encouraged. Rather than comparing numbers (My strength is 403.54. What's yours?), players should instead be comparing their stats by various feats they can accomplish in-game (I can pick up this boulder and carry it from here to the end of the road without getting tired.) Find ways to make character development and abilities more interesting than a simple linear progression through a spreadsheet full of numbers. Allow the players' imaginations to fill the gaps
6) Every action has a reaction.. and repercussions - Factions are actual choices, not just a farming opportunity for special rewards. If you're doing things to help out one faction, it's very likely that you're pissing off their enemies at the same time. Wholesale slaughtering of all the wildlife while leaving their bodies to rot? Nature-lovers are likely to be unhappy. Killing a farmer's dog? You can bet he'll be unhappy! Strip-mine the mountain? Maybe you cause avalanches, screw up the water in a nearby river, or release a Balrog! I want the players to put some thought into what they're doing, why they're doing it and what might happen as a result of their actions. Everything is inter-related.
7) Horizontal growth of individual player power - No levels, Skill-based, with a very complex, intertwined tree structure (Eve Online is a good example of the direction Im thinking). Players gain new powers to bolster their arsenal, rather than simply scaling up damage and hit points to the point that a developed character can lay down on the ground and take a nap while a new character can swing at them all day with no chance of killing them. Character skills and abilities definitely have an effect on your play experience, but character uberness comes mostly from player skill, not items or levels. Everyone has the ability to defeat everyone else given the proper circumstances.
So there you have it. I've tried to limit my list to fairly broad core concepts rather then detailed mechanics, and also divided in such a way so that they're not reliant on each other. Of course, in my mind, they all form part of an integrated whole, but I think that any of them could be taken and implemented as a single entity within an MMO.