I think there are two important factors to the UO experience:
1] As Borg says, you can experience everything. Yep we can take you out to see an Ancient Lich Lord the first day you're born. And then you'll die. But at least you get to *see* the damn thing and be shocked and awed.
2] Travel is a breeze. The functionality of recall and gate magic again allows you to *experience* all the environments the designers spent so much time on.
I for one enjoy skill based games as opposed to level-treadmills like everquest. So I enjoy that aspect of UO and I enjoyed that aspect of SWG.
The other feature of UO which drew me back was the ability to SEE everything. No tedious running for hours (though I do sometimes to find new things). If I feel like hunting, I recall to a hunting spot and HUNT. I don't spend an hour getting buffs from some doctor, then 15 minutes waiting for a ship, then 20 minutes driving a speederbike across a barren and computer generated countryside.
Its a relaxed game, there's plenty of things to do and plenty of creatures to see and places to go. And as I said above, the best part is you can actually do it all without grinding out skills beforehand! You won't necessarily conquer everything, but at least you can check it out!
-LizShue