Why did I enjoy UO in its late-beta early-release stage, then, as a complete non-griefer?
Granted, I was a lot younger back then, so this may all simply be rose-tinted goggles.
But here's what I liked about it:
Player housing that wasn't too widespread. This was before every buildable square inch of the gameworld was covered in castles and houses. The wilderness actually felt like wilderness. There were birds flitting around, and then all of a sudden you hear an ettin roar. Rut roh! (Compare to when I left it, about a year later: running between houses.. between houses.. argh what's with all the houses.. hey, a tower with "ASS" spelt out in cloth on its roof..)
First entrepreneurial act: saving up enough money to buy one of those rare dye tubs in the trinsic tailer's shop, and proceeding to sell customization to other players who missed the spawn! Then again, the guards were broken that day in beta, and a group of hoodlums had set up shop at the south entrance. (beta)
Hanging out at the Yew Trading Company; one of the first guilds on Great Lakes to get a house with a forge in it placed it in the field at the crossroads just south of Yew. They took & delivered orders through the window. Occasionally PKs would attack, so they formed an alliance with a more combat oriented guild. They'd pay guildmembers to sit around outside and protect their clientele.
A true sense of "danger;" every time a stranger came on screen I'd hit my all names hotkey. If they were red, I'd run the other direction as fast as I could. Running away from those big bad dread lords was fun! It got my blood pumping! Heck, I'd just bought some new platemail from Lilo! Compared to yawn, another instance...
Had one character who was perpetually grey. Had studded leather armor of the best magical rating, along with an imminently accurate bow of vanquishing. And he was a GM archer/tactician/hiding. PKs and NPKs alike would try to kill him. He'd either run and hide or kill 'em outright. What kept him grey was if he saw someone kill an animal (bird|rabbit|hart|bear) and not skin it, he'd run em out of "his woods." After giving them ample warning to gtfo our quit it.
The Orcs who set up base at the orc camp southwest of Yew. They were badass, and humongous. Occasionally they'd set up camp along the road and demand tribute. Occasionally they'd get attacked by people who thought they were badass PvP guilds.
They almost always lost. There were almost always ten or twenty orcs hanging out at the fort. Sometimes a lot more.
Their Drinkee fests were freakin' great.
That's the kind of content you can't get from WoW. Or any other carebare MMO. You don't even get that kind of content with Eve (though you do get truly righteous massive space battles, which are kinda cool I guess). Heck, even a primarily PvP game like DAoC didn't get content like that.
What's missing? Here's the attributes UO had that garnered more of that kind of behavior than any other MMO to date:
1) Free-range PvP outside of towns
2) easy ability to tell if a PK was a PK on first sight
3) "stuff" was relatively easy and cheap to come by. Lost a set of armor? meh, you probably have another couple sets sitting in the bank that are just as good.
4) You didn't have to go out grinding a treadmill to get to a state where you could comfortable interact with the rest of the game. It took 3-4 days of heavy playing to get a solid character up and running.
5) there were craptons of "useless" items that actually showed up when you dropped 'em on the ground. Bones, rugs, mugs, clothes, everything. Heck, even "beef jerky" (too bad they had to take that out after they released in Germany)
6) It didn't force your playing into a paradigm. Instead of being an amusement park with clearly marked lines and rides, it was an adventure.
7) At the time, it was something that was brand spankin' new. Sure, Meridian 59 and other MUDs around had done similar stuff. But none of it had the mainstream appeal that UO had.
Of course, in parting, my favorite thing that happened in UO happened before release, when things were still pretty buggy:
To set the stage: Trinsic. The guards are bugged again - none of them are around. Everybody's running around in their newbie equipment, because characters just got wiped an hour or two ago. All of a sudden, a naked guy comes running in the west entrance yelling "ETTIN! TROLL!!!!!" ... everybody runs towards the entrance to see it. Then immediately backpedals. There's a couple trolls, an ettin, and moseying on behind them: a dragon. Straight into Trinsic. People hide in houses. People hide on roofs.
The carnage was unbelievable.
The explanation even moreso:
Back then, they had an ecology system whereby creatures had to, well, eat. Cougar eats pig, orc eats cougar. Unfortunately the only mobs easy enough for the glut of newbs to kill were birds and rabbits and rats. So eventually the bigger, badder mobs got hungry. And started wandering farther and farther afield.
Until they got to Trinsic. And had their gluttony sated by the hordes of noobs.
And then there was witnessing Rainz murder Lord British. That was pretty fun, too :)