I'm glad that "Meaningful PvP" was singled out. PvP is important to me and its one of the things that Ultima Online got right that vast swaths of MMOs since have floundered with (in my opinion).

In parts of the world (Felucca[1]), UO was a rare RPG where anyone could kill anyone, for any reason, but with the repercussion that they would be branded a "Bad person" (visible with a gray or red name instead of blue). Stealing from good people corpses also did this. Anyone can attack and kill bad persons, and if you killed even more people you were a murderer and it took a very long time to return to normal.

You want to use super awesome powerful gear? None of this sissy MMO stuff. Die and you lose it, and your enemy (or his enemy!) gets the spoils.[1]

Those are the two criteria by which I think "meaningful PvP" should be judged:

1. Can I kill anyone? (ie, in WoW you can't kill 90% of the people you see, they have to be in an arena/faction/alliance).

2. When you kill someone do you actually win something? (preferably something of theirs), and when you die do you actually lose something (preferably the equipment you risked to try and have a more favorable fight).

Ultima and UO were very big into morality and moral dilemmas, which is a thing I love in both single and multi-player games. Some of the Ultima games featured very interesting choices, like whether or not you should kill "evil" (possessed) children or leave them alone.

In UO, if you did have a "bad person" title and wanted to know how much time you had left before you would be considered blue again, you had to type:

"I must consider my sins"

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The other problem I have with a lot of MMOs is that the power of your character is simply how much time you sink into the game. Essentially, MMOs are games that reward wasting time.

UO had so much more than that. The class-less system definitely helped, and UO was a game where treachery and sneakiness really paid off, if you wanted them to. Lots of ways to nearly instantly kill or entrap people lead to a lot of very exciting plots where guilds might be laden with spies. Absolutely nothing like the limited PvP found in many modern MMOs.

In a lot of ways UO was the Diplomacy (diplomatic back-stabbing board game) of MMOs. And it was great.

See also outworlder's wonderful comment explaining UO 90 days ago[2]. Also considerably interesting was the economy of UO[3], which was wrought with a good deal of experimentation.

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Also, if you played UO, you'd know that property taxes are a wonderful idea. So many people land-rushing to get the largest properties possible, who then sat on them and never built anything! (Or never played, while newcoming regulars had zero chance of ever finding a home at a reasonable price)

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[1] In the beginning there was only Felucca. And an insurance system was added later (2005ish?) where you could pay a certain amount per item to not lose it on death. Both were attempts to make the game less harsh. Like a lot of later patches, this was unpopular with older players and popular with newer players. Over its lifetime, UO did a lot of things to make the game world less cut-throat, which will always be controversial. Some realms rules stayed more "hardcore" than others as a compromise.

[2] http://news.ycombinator.com/item?id=4890513

[3] http://news.ycombinator.com/item?id=4890481