I keep seeing disucssions, arguments, threads, and really wrong statements about these. I figured, since most forum goers probably didn't do them in the first place, perhaps they should see what they actually were. I've given links to a few things people might not know off the top of their head and or long lines such as Black Temples which IS heavy on quests. There are a few notes and two interesting points of reference listed.
Also, for quick reference
Karazhan Attunement : Removed with 2.4.0 (25th-Mar-2008)
Serpentshrine Caverns Attunement : Removed with 2.1.2 Hotfix (19th-Jun-2007)*
The Eye Attunement : Removed with 2.1.2 Hotfix (19th-Jun-2007)*
Mt. Hyjal Attunement : Removed with 2.4.0 (25th-Mar-2008)
Black Temple Attunement : Removed with 2.4.0 (25th-Mar-2008)
* "After a lot of thought and deliberation, we’ve decided to remove the attunement requirements to enter Serpentshrine Cavern and Tempest Keep: The Eye. While many of our attunements in the Burning Crusade have been good progression checks, a few of the attunements have turned out to cause unnecessary stress on guilds either doing the content or attempting to do the content. With Black Temple and Battle for Mount Hyjal thriving, we want to encourage (rather than prevent) new guilds and raid groups to attempt Serpentshrine and TK. We are going to leave the current attunement quests in the game so that players can still engage in the challenge and the lore of those quests should they choose to. At a later point, we are considering adding a final reward step to those quests as well (that way those who have already completed them would not miss out on a *new* reward)."
-Get to Karazhan
-Do 2 quests in front of Karazhan given by the Npc there
-Npc gives a 3rd quest which sends you to Dalaran
-At Dalaran you are given the quest for the First Key Fragment (Shadow Labyrinth)
/This required access which came from Sethekk Halls unless someone else had the key
-After obtaining the First Key Fragment, return to Dalaran
-Accept quest for the Second Key Fragment (Steamvault)
-Go to Area52 for quest Warp-Raider Nesaad
Warp-Raider Nesaad is a 9 step quest line which is skippable if someone had a key to Arcatraz already!
-Ogres in the area are hostile however unless you completed quest lines in Blade's Edge Mountains
Serpent Shrine Cavern
-Enter Slave Pens Heroic (required Key which required Revered prior to 2.3)
-Accept quest given by Skar'this the Heretic
/Quest requires player to kill both Gruul the Dragonkiller & Nightbane (karazhan)
-Return to Skar'this and complete quest
Pre-req : Required Flying or Warlock summon to enter the raid.
This was the first "long questline" Surmised still best off of WoW Wiki here
Important thing of note for this particular line. The Tempest Key could be started at 68. This allowed for leveling progression throughout the quest line.
-Travel to Caverns of Time
-Retrieve quest The Vials of Eternity from Soridormi
/Quest requires two kills for two drops ; Lady Vashj (SSC) and Kael'thas Sunstrider (TK)
-Return to Soridormi and turn in the quest
*note: Battle for Mt. Hyjal required Attunment for All Raid Party
BT had the longest quest line located here
Another interesting lore based quest line which too could be started at 68 for the first half, netting xp and rewards.