I used to have a server or two running under Sphere Server(if anyone even knows what that is). But it had its own modding language instead of one regularly used, it was closed source, and I was young and totally inexperienced at that time anyway. I think I'll be using UOX this time around; Open Source, moddable with JavaScript, etc. Awesome.
There's too many to advertise.
If you want to know precise details, there's a lot of info at
http://uo.stratics.com/.
basically it's game with hunger and other stuff (scriptable, i.e. some servers have tiredness and body temperature/sickness).
It features simple stats system (strength, agility, intelligence -> health, stamina, mana ; also stats affect verious things, i.e. strength -> melee damage, agi -> attack speed etc.).
But it has nice skill system: 54 vanilla + expansions/scripts.
The skills grow with usage, no levels etc. However many shards use class system with skill restrictions and class levels, but I prefer original free skills.
There's a huge world with islands, caves and dungeons, different biomes from deserts to snow plains etc. Hundreds of different monsters and nice quests (if you have nice staff add to this nice Game Master-driven quests like attacks of monsters on cities and quests for artifacts of power).
It has complex magery system with different spells from 'magic arrow' and 'create food' to 'summon demon' and 'polymorph self'.
There're various jobs and professions avaliable to you: you may be a ranger, or warrior, or mighty wizard, or sneaky thief, or mere crops-grower, or everything-in-one. However crafts are not too easy to study, i.e. you have to smelt + forge A LOT of various daggers, bucklers and helmets to reach the Grandmaster status. Sewing is even harder, because you should grow cotton first/breed sheep and get wool from them, then process it at loom and cut it on pieces of cloth. However you just can buy it.
PvP system is balanced (usually, however it strongly depends on server scripts), strong guild system, guildwars etc.
Own property: you can build your own houses and decorate them with crafted items/furniture, in earlier versions you had basic set of buildings from "toilet" (1-room house) to huge castle. However in later versions there's a custom structures feature: you can make your own house using set of objects (walls, windows, teleporters, ladders etc.) which leads to some very unique buildings (like skull-shaped dark temple or an Elven glade).
Boats, mounts (from horse to dinosaur-like zostriches and magic summons).
Bah, too many things to explain.
Plus "Omigod B@D GR%PHIXX" for many young players (but whom do I say it to... you play DF), but I like that 2d-style, it's in my heart and I won't exchange it for anything (there's a pseudo-3d interface with 3d objects but it's bad for me -too hard to "read", guys who can't get used to graphic tilesets here will understand me easily).
Screenees:
1) OMG naked people! xD
2) "gandalf"
and adventurers vs Cyclops on a bridge
3) Example of custom house and 3d interface
http://www.kingdomreborn.ru/images/uokr/aboutKR/about6b.jpgP.S. mag eis polymorphed into the red dragon here.
4) Equipment and characters (this image really shows just different "desktops" - visual styles for 2d version.