This required that another player literally GIVE me his or her items. As in, I could run off with them if I wanted. There was no trade window that allowed me to repair their items without actually taking them. They had to trust that I would give their items back after repairing them.
I found this absolutely fascinating (and still do) for multiple reasons:
1. Over time you built up a reputation of being trustworthy. At first people would only give a smith they didn't know crappy items, but as you became more trustworthy and well known they would start to hand over their rare items for repair. I was literally building a reputation in an online game, in a real community, for providing a quality service (think about the impact this likely had on a 14 year old, who at that time had no idea what having a job, or building a reputation, was like).
2. Items HAD to be repaired in UO, so this wasn't something players could get around (unless they wanted to train up that skill themselves, and you were limited on how many skills you could train to their max level at any given time). You simply had to trust a blacksmith in order to repair your items. After players came back to town after long sessions of dungeon crawling and killing monsters (in many cases because their armor was in need of repair), they would funnel back into town and the first stop was generally to get repairs.
3. For whatever reason everyone stuck to a tips based system. I don't remember a market rate at any time for repairs that players set as a community. It was just assumed you tipped your smith.
4. The better armor/weapons you personally wore while smithing, the more likely you'd get customers who had better weapons/armor, resulting in higher tips (because they likely had more money). It was my first experience with advertising and building a personal brand.
This is just a taste of why UO was such an amazing game and social experiment. It's a shame nothing has come close to recreating such an experience (EVE Online being an exception, but it still falls short in many ways).
If I had to pick a second fondest aspect of the game, I'd probably say Rune Books. You could "mark" runes in the game with a spell, which coded that rune with the location you were standing in when you marked it, which you could then place in a book. You could then travel back to that exact location at will, at the cost of a spell and its required reagents (hell, the reagents system was amazing as well!). Players made Rune Books of all the towns, dungeons, and crazy hidden locations (or their own homes, favorite shops, etc) for personal use, but also to sell so other players didn't have to make them on their own (as it would require a significant amount of traveling on foot to do so). I remember being very fond of my Rune Book collection.
What a game.