I started playing UO for real several months back because of the memories I had playing on one of my friend's accounts.
Back in 99' or so, one of my friends and his older brother had an account. When I'd go to his house, I'd make a newbie thief on my friend's brother's account so I could play. This one time, working as a team, I was able to loot a house key and pass it off to my friend. He booked it, and what ensued was the most intense thirty minutes in gaming I've ever experienced. When it was all said and done, we had us a new house. We weren't all bad, though - we took a few things, gave the player back her key, and that was that.
The week after that, we stood on a bridge as we saw a player steering his boat under it. When he got under the bridge, we jumped onto his boat, piloted it out to the middle of nowhere, stole his sword, and killed him while he tried to fight back with a fishing pole. Granted, not as epic as the previous weeks' adventure, but it was this open ended play-style that attracted me.
Oh. And then there was that one time him and I somehow ended up with a flying boat. That was what finally made me settle down and buy the game. With this flying boat, we were able to fly up to large towers, jump onto the roof, and loot them. Granted, it was evil, but we ended up with ( what I now realize to be ) some crazy, crazy rares. I want to say they were server birth rares or something. I remember there was this large stone table that was made up of six pieces or something, and a bunch of other miscellaneous items.
So yeah...I was drawn in by open ended game play where anything was possible. Nowadays, this stuff isn't really possible, and even if it were, things are different now. Back then, the only thing you lost if someone robbed you was GM armor and potions, things which could easily be replaced, unless it was a true rare. If houses could be robbed nowadays, it would suck big time, because stuff is so much harder to get.