What do I mean by "leading?" Simply that a design idea implemented into an mmorpg that turned out to be win for one reason or another. This does not mean the mmo itself is all good. It just means they got at least that thing righter than anyone else.
MMORPGS have been around since before Ultima Online, although nobody paid them any attention before UO. For this reason UO just screams leader. And it was, and still is, in many aspects. Out of all the MMORPGs i have played, UO was the most sandbox like.
Not only could you pvp or pve (both were very fun), but they were mixed together perfectly (until later when everything was ruined).
And yet there were many other things you could do that couldn't be designated as pvp or pve. For example:
You could build furniture then use it to block bridges in Vesper and piss people off getting them trapped by throwing down more furniture as they try to cross.
You could hide behind a rock and send messages to people to freak them out before you were able to highlight text to attack and double click to see who it is (lame and counter intuitive).
You could gate monsters around to create chaos before they made it so you couldn't. You could even gate into town before they made guards insta kill them. UO got too carebear, but it still showed the world sandbox concepts that no one has matched, not even darkfall, well over a decade later.
EQ certainly was a leader in PVE. It was also a leader in exploration. The reason was not only was the landscape very well designed and things all looked very different from one side to the other...but it had a great immersion factor...you would have to take a boat across an ocean to get to another continent and it was a pretty long ride -- and dangerous.
If you fell off you would be stuck around islands with high level mobs (sirens! getting killed by hot fish women, awesome!) and have one hell of a time finishing your journey. There were even secret areas in those trecherous waters that you needed to go to, to get your spells.
But that was just it...there was content everywhere and no map or radar to really know where the hell you were going. You had to make notes and maps and remember landmarks.
And what made it a leader was on top of all of this if you died all your expensive gear would be lost on it. You would have to pull it back to safety...oh yeah and did I mention TRAIN!!!! was badass? This meant you wouldn't be able to run around high level areas and check em out and get bored of them before you were ready for them. It kept mystery in the game.
Asheron's Call for some reason had a really fun grind.
SWG had that awesome harvesting feature and their player skill exchange (doctors buffing, dancing in the cantina etc) was pretty great. I didn't get heavy into the pvp but keno seems to like it so it must be good, too.
Dofus would be a leading mmo in the field of loot tables. Their rare drop items ruled the game and there were so many...so many quests that would unlock new emotes (darkfall should have this) many of which you couldn't do solo and which had puzzles in them. (why does darkfall have no puzzle rooms?)
Eve Online figured out how to do time cards and how to control gold selling with their plexus cards. Basically, it's an item you can pay money for that you can trade to other people. In darkfall it would only work if it was a no-drop full-trade item.
The idea is that you pay someone a time card (paid for by rl cash to CCP, the company that runs Eve Online by you) and they give you in game credits.
The reason this is brilliant is that people sell gold no matter what. It's like drugs. When eve made it legal to do it THIS way they made it so all the money flows through their hands. Whenever people trade gold this way it means another account has 1 more month of darkfall...
And CCP doesn't pay for their month. Whoever bought the time card does. This keeps poor people in the game, too. People who would be very active in the game since they'd have to be to get the gold to buy a time card from someone.
One thing Darkfall does right is the stealth factor. The importance of using terrain and hiding behind things...crouch walking to be quiet and how useful it is to use 3D sounds. Since it's actually like a fps unlike games like WoW that are more automated with auto-targeting tracking spells etc you actually need to be able to hear positions of enemies.
I'm sure you know of some leading mmorpgs. Can you think of any others?