Yeah, great times indeed.
Open unrestricted PvP, real "danger" when leaving the towns in case you lost all your belongings. Craftable weapons, armour, clothing, furniture. Dyeable weapons, armour, clothing, furniture. Personal ships, personal housing, personal mounts and pets. Skill based system. No classes, no quests, no levels, no epic loot, shared dungeons, no level indicators on mobs so you could tell if you could kill them or not. Hell, you could even kill a player, take his belongings then skin him with a knife, then cook his skinned flesh and eat it.
The game just had it all, and all in the gameworld. No instancing, no zoning, no loading screens. Loved it. Really showed what can be possible in an MMO if the devs would think a little more outside the box instead of just reproducing re-skinned clones of previous MMOs.
Will always remember my years in UO, probably the best gaming experience I've ever had