I have to repeat it: The UO Houses were just fantastic and actually meant something. Being in a good location made it easier to sell your stuff on your vendor. This is what I miss most about the game...
Asheron's Call's guild system - specifically, vassals. You get invited into the guild by someone and through that become his vassal. As you gain exp, he will gain a small share of what you make (though you don't lose any). As you progress through the game and get to a higher level, you'll start inviting people of your own. Others will become your vassal, providing you (and those above you) with exp.
It's a great incentive for people to assist newbies and get them involved in the game. I can't believe this has never been copied
