Ok I've been thinking about this for a while now and also I just finally quit Final Fantasy XI, so I've come up with stuff that makes a good MMORPG. I COULD just point at World Of Warcraft and say pretty much that but that's not completely true either. It has the majority of them but it is missing some things.
1) Community - This is first on my list because it's the most important. When I played Ragnarok Online(most of you probably haven't) on a private server I had nothing but fun because of all the friends I made... until they migrated to other MMORPGs and then I got bored. A thriving community reminds us that there are people to play with and plenty of good times to be had, any WOW player can agree to that because of the fact that a good community and friends makes grinding less tedious. This is where Final Fantasy XI choked because it's community was small and worst of all the language barrier made life miserable when playing with people who can't speak your language and even it's translator option was pathetic.
2) Economy - Another high important thing for MMORPGs that Final Fantasy screwed up on release and screwed up with an update back in 08/09. WOW has you earn gold reasonably easily which keeps it's economy strong and no one is broke for the most part. A good drop rate of crap and even gold rewards for killing monsters is a good way to keep money flowing. Final Fantasy and Ragnarok screwed up here making your income revolve around what you picked up and in Final Fantasy XI's case it dropped the price you could sell stuff for while keeping the cost of items to buy the same. Auction House's works wonders in MMORPGs but the problem with them is that a lot of people are asking too much for items that would be easier to quest for instead of grind for cash.
3) Leveling - Again Final Fantasy XI trips up here... maybe this should have been "lets point out Final Fantasy XI's mistakes". Final Fantasy XI Forced you to party up to gain levels by making monsters stop earning you exp at certain levels in areas and making it impossible to solo in areas where you could earn exp. Level 30 was the max level you could get on your own with grinding before the enemies became too strong to solo. This is where WOW shines it's brightest. In WOW you earned exp from quest which are abundant, monsters always give at least 1 exp and you can solo to the max level. If you wanted to level up in Final Fantasy XI you had to party and pray that they wanted you in the party because some jobs classes are in high demand for a party and others weren't. Sure they added a level sync ability so even the high level players can party with weaklings to level up in low level areas but it still made life miserable.
4) Quest - Every MMORPG has quests, but how many are completable without help? DCUO shines here since every quest can be done solo. Yes some quest should have a party or guild focus, but ragging on Final Fantasy XI 90% of the quests require a helping hand and all the best equipment and items was obtained like this. Also the reward from quests need to be better suited to the difficulty instead of a garbage item from a 6 member party quest(again targeting Final Fantasy XI).
5) Abilities - Again I'm taking another chunk out of Final Fantasy XI here because the way the managed abilities was dumber then remaking pong. In Final Fantasy XI you had to quest and buy abilities for all of your classes, which mean 1000's of hours of grind and questing to be able to get them. In other words you could be a max level classes with almost no abilities. Imagine a LV 60 Red Mage with it's highest spell being LV 36. No one wants to party since you have no useful magic and on top of that you have no Gil to buy any new ones at the Auction House. This is where almost EVERY OTHER MMORPG OTHER THEN FINAL FANTASY XI gets it right. See in all the other MMORPGs you usually get skill points which you use for your abilities so you can learn what you want and create the builds you want.
6) Multi classing and changeable classes - The ONLY thing Final Fantasy XI got right! Some of you have made characters in other MMORPGs and have always wanted to keep them and change their class so you don't have to manage characters. Final Fantasy XI gave you a character and gave you about 20 job classes(most you have to quest for) and each class had it's own level. This was a great idea(except the job questing) and being able to set a sub job class to have abilities and stats from another class added on made life so good. Red Mages with Ninja sub class were common in Final Fantasy XI for cash grinding and White Mage/Black Mage combos were popular in parties because of extra MP and improved spell versatility. All other MMORPGs never to this which is a shame since WOW could be greatly improved with multi class and class changing, heck I'd play it again if it were the case.
7) Combat - They all do it right. I can't come up with anything that needs improving there.
8) Universal Translator - I can't stress this anymore, every MMORPG needs a proper translator inbuilt into the game so whatever you type comes out in the language you speak. No keyword crap, no semi translate crap, just a full proper translator where you type it, they read it in their language. NO MMORPG DOES THIS AT ALL! It should especially be done if you pay a subscription fee.
I think that covers it for the most part. MMORPGs have plenty of room for improvement and adaptability but sometimes they don't want to or in the case of Final Fantasy XI be made well and they destroy it over the years. I think WOW is the closest to the perfect MMORPG but there are things it's missing. Will we ever see the perfect MMORPG? Probably never. All I can say is have fun with what you find and if they change something for the worst, force them to change it back.