I realize this has been done before, but with the advent of TBC means that a lot of this content is at risk of getting "lost" in the grind to 70. It would be great to have a high-quality resource explaining all the long (more than 10 steps), hidden (not often done), or just exceptional-quality quests in WoW.
I didn't have any luck finding any resource like this on wowwiki or via Google, so I'm going to start one. I did find allusions to lists like this in many places, including a couple links to old official forum threads, but the threads are dead and other lists are nonexistent. I'm also leaving out the longer raid questlines, notably the AQ quest that starts with the Broodlord's head, because it's unlikely that anyone will complete that ever again.
I'm also specifically leaving out TBC questlines because I haven't done them yet, and the number of extremely long lines is far greater in TBC than it was before. Additionally, most of the really long questlines in TBC have a purpose (like key for something, earn huge amounts of rep, etc.), which isn't really the purpose of this list.
Please make an effort to post links to places that detail all the steps in the quests in question, so that questionable inquiry is kept quiet; I'd like this thread to be relatively high-quality and contain relative dense information.
1. You are Rakh'likh, Demon (aka, the Fallen Hero questline, or the Badlands Demon questline)
This is one of the longest, most well-written, and most interesting questlines in the game. It's one of the few quests that really convinced me that a nonliner RPG can work as well as a linear one. It has you running all around the world -- from STV to Azshara -- and you end up feeling like you're really part of a larger world, doing something incredibly fun.
The rewards, while once very good for the level, are now far outstripped by quest rewards in TBC, but that doesn't mean the questline isn't worth doing. The final boss is a challenge for any PUG, and even an experienced group with no fore-knowledge of the fight will struggle for a wipe or two. This quest is definitely worth grabbing, even if you're just a bored 70 looking for something to do on a Sunday morning.
2. Linken's Questline
This quest starts when you bring an item found in Un'goro to an NPC named Linken. The line is 13 steps long and ends with a pair of fairly decent rewards (particularly for a Paladin). The questline is detailed in the third comment on this Wowhead page.
Again, the questline gives you the sense of being part of a larger world. All the allusions to Link from the Legend of Zelda are just icing on the cake.
3. Onyxia Key (the questline for the Drakefire Amulet)
This is a huge, mega, mammoth questline that is different for both factions. It is detailed extensive on this WoWWiki page. Any new player should endeavour to complete this at some point -- the storyline that this quest unfolds is a central part of the Warcraft lore.
4. The Missing Diplomat
This is a long, Alliance-only questline that I can't really comment on (because I've never done it), but I've heard many friends who speak fondly of it. It takes you all over, though has mediocre rewards. The end is also a little anti-climactic... in the sense that it doesn't really end, the quest line just finishes. I place it in this list because it's long.
The line starts with these 11 quests, continues with these four, and concludes with this reward.
5. The Tirion Fordring series
This questline gives a lot of backstory about the Scarlet Crusade, and it is the first long elite questline that most players will do. The rewards are spectacular for their level, and the questline is extremely fun, involving disguising yourself as an enemy and walking into their castle. The line starts with the link above and ends with this quest after watching the existing Scarlet commander get ganked. Well worth running through, even at 70, just for the storyline.
6. A visit with Greatmother. See also the WoWWiki page. Part of TBC, this questline has you involved in Thrall's past. I would assume there is no Alliance counterpart. If you could the entire series and the series required to gain access to it, it's well over 30 steps long from what I can gather. (Thanks to Nite_Moogle)
7. The Archmage's Staff. See also Archamge Vargoth on WoWWiki. Might also check out Malevolent Remnants, which is available after completing part of the Vargoth questline. (Thanks to chase)
8. Screecher Spirits. See also the whole questline. This questline is basically an introduction to Zul'Gurub and Hakkar as a fixture of lore. (Thanks to missiletoad)
9. The Scythe of Elune questline. This looks like a really interesting bit of game. It essentially explains a small piece of the world -- some other questlines listed here try to take a larger view, but this is very focused on a single piece of mythology and unexplained circumstance that is part of the world at large. Looks well-worth checking out. (Thanks to Linnet and several others.) Maledict's hypothesis on these events is worth posting for those disappointed with the end of the line:
The reason this questline doesn't end is because they messed it up. It happens occasionally. If you've levelled a Horde character, you'll know what was *supposed* to happen - Arugal was supposed to end up with the Scythe, as it's the only explanation for everything in Silverpine. (It's revealed that Arugal has very little magical power - he uses powerful artifacts to simulate casting spells). Presumably, at some point the Ashenvale / Darkshore questline with the Scythe was supposed to end up with the Alliance players getting a quest to go into Shadowfang Keep and kill him to retrieve it. However, for whatever reason that never panned out, so the Alliance quest line just ends and the Horde get a lot of lore relating to how useless Arugal is and how it's a mystery to anyone how he could actually cast the curse, without ever making the link between the two.
At least, that's the best explanation we've been able to come up with for the discrepancies for both factions regaridng the scythe and Arugal, and the fact it's never been fixed. (And it is odd how the Alliance don't get any general quests for Shadowfang Keep, one of the best early dungeons in the game).
Should also check out the GameSpy list:
This page in particular is a list of long-ish questlines from level 52-60:
(Though not all of them are really "excellent", in my not-so-humble opinion. The Scholomance key quest, for instance, isn't really that interesting.)
I've ignored several quests that were mentioned because I don't feel like most of the population would get anything out of me adding them (the Scepter questline, the Pallie epic charger questline, a number of the maller one-step quests suggested by alienangel. But, I will be adding a page to the WoWWiki about these quests soon. I'll post the link here when it's done. You can feel free to put whatever you want on that page. If I have time, after I make the page, I'll try to include all the stuff listed in this thread. Odds are if one person has remembered the quests this long, they're probably worth mentioning to a larger audience.
There is a WoWWiki page now: