I no longer play UO, but did as recently as 2 months ago. However, I haven't enjoyed PvP in UO in many years (since the world split).
What I *did* enjoy about PvP back in the old days was the social impact it had on the world. Sosaria was a land in turmoil. There were monsters in the land...but the worst of them were the evil human beings that roamed the countryside looking for innocent victims. Those "monsters", those were the ones that posed the biggest thread...and the greatest challenge of all.
For me, and my guild, fighting PKs was our role in the game. We were defenders of the innocent...Protectors of Virtue. Sure, there were trolls and ettins and such...but those are just dumb creatures. They were not genuinely malevolent. In fact, most of the creatures you would come across in the wild were merely out foraging, hunting for food, or just dwelling in their natural habitats. But PK's ... there was a force of real evil. These were sentient, intelligent beings that made a conscious decision to control certain areas of the land. And unlike lesser creatures, like ettins and liches and such, they were not lumbering idiots that could barely defend themselves. Far from it in fact!
It was the concept of good vs. evil, instead of good vs. ambivalent, that really got me interested in PvP. Once that was gone, so was the allure of PvP in Ultima Online for me.