In the PvP areas you could effectively run a toll bridge by laying out a road block, and fight those who tried to just push past.
A major difference with later MMOs was that (originally) when you died, your possessions stayed on your corpse. You would have to wander off as a ghost to get resurrected, then somehow get back to your body to get your kit back - a tall order in the middle of a dungeon!
Without any form of global chat, the best you could manage was party / guild chat, but often this wasn't enough to get the help you needed. It became commonplace amongst the community to build a metagame community - you might not be in the same guild in game, but you were friends in the forum or in real life, or perhaps you met eachother in game and wanted a means to stay in touch. This posed a problem - no player-to-player system was really in place, and generally minor roleplaying was common - exchanging email addresses felt wrong and out of character. Thus the "pigeon number" was born - ICQ was the staple communication tool. When gAIM, the open source AOL/ICQ messenger (and others), renamed to pidgin, I can't help but think this was deliberate.... ;)
The huge downside to these interesting mechanics and heavy freedom was the opportunity to grief. Kill someone, loot their stuff, ransom it back. Let monsters kill someone, loot their stuff, ransom it back. Tell a ghost you'll only resurrect at their corpse and... you get the idea ;) I always was a big fan of some of the griefer stories, even if I didn't agree with their ethics - Belan being a prime example http://www.askcorran.com/belan/episodes.html
Ultima Online was classless - you developed skills through their use. This also lead to some of us who roleplayed beggars trying to build the ultimate in useless characters - every single skill point up to the cap spent on skills that offered in total no benefit in terms of gameplay mechanic. Begging in UO was a real artform, and there was so much fun to be had roleplaying a widow and seeing what people would offer you.
And finally, these mechanics that few other MMOs have ever implemented? Ultima Online inspired two UK students to build their own version, which became Runescape. Runescape inspired Wurm Online. And one of the Wurm developers, was Notch of Minecraft fame. For all UO's faults, it is truly an important piece of gaming history