Here is a list (a few years ago) of the closed quests that I noticed. I am sure there are many more nowadays (I order them by creator, if I can remember):
Mats: Explore Mordenkainen
Profezzorn: Inspire Profezzorn, Help Escher with his Drawings, Rob Death, Find Madame X's Crystal Ball, Make Scumble, Nannymud's Bureau of Investigations, Find Merlin's Hidden Treasure, Go 18 Holes on Nannymud's Golf, Help Nellie to Find her Cat
Milamber: Wake the Sleeping Dragon, Save a Valley from Flooding
Garion: The Belgariad Quest
Darick: Black Wizard
Tanya: Tanya's Prison,
Make a Present for Dr. Strange (for Maggs), Find Tanya's pearl necklace
Aphrodyte: Find the Treasure in Axil's Funny House
Slater: Catch a Fish and Cook it
Kerish: Help the Tortoise
Earendil: Wand of Creation
Jojina: Build Your Own Castle
Akasha: Akasha's Bonsai Tree
Moonchild: Climb the Tower of Games, Help the Old Druid Slay the Lumberjack, Recover the Lost Crystal of Life, Destroy the Great Evil Threatening the Land
Ppannion: Explore the Hidden Desert
Nib: Build a New Guitar for the Old Musician, Make so the citizens of Bogor can return home, Make a Wish
Gogo: The Magical Topaz
Aita: Put the skeleton back together
Tulkas: Find the Ring, and seek its destruction, Riddle Hut
Padrone: Padland
Charmangle: Quest for the Soulstone
Mordor: The Magic Book
This is extremely incomplete and does not list the quests before my time, such as Badman's Bicycle quest or Raistlin's Tame Wild Ones (or does that mean those are listed now, too) and any quests since I stopped playing my mortals all the time (circa 1997 - or so...there WAS a spurt of playing from 1999-2001).
/Snafu