Before the new alliance race was announced for The Burning Crusade, a supposed "leak" said it would be the people of Gilneas, transformed by the crazed wizard Arugal into bestial worgen. The new race was actually the draenei, but two expansions later we did get worgen... with the same origin story as the faked leak.
Boss Mida, found in the Goblin Slums of Orgrimmar, is a Shout-Out to the "Trade Princess Movement", a thread from the old community site requesting a female faction leader for the goblin race. Official faction leader Trade Prince Gallywix doesn't appear ingame after the starting zone, and Mida explains that she's been running the whole show while he's off picking wallpaper for his pleasure palace in Azshara.
Every current faction leader (or faction leader trio) have a short story written about them. With the exception of Lor'themar Theron, who's short story is actually the 2009 Writing Contest winner "In the Shadow of the Sun". The writer of that fanfic, Sarah Pine, also wrote the Garrosh short story.
Demand Overload: The launch of Warlords of Draenor was plagued by unstable servers, latency issues and massive login queues. There were several causes for this, but the primary one was the huge numbers of people trying to log in at the same time and bottlenecking in the same zone. In the words of a Community Manager:
We obviously expected an increase in logins, and prepared for well above what we were expecting. The actual amount is far above even that.
Hey, It's That Sound!: Many enemies, especially humanoids, make a low dying moan that will be familiar to players of DOOM; it's the death sound of a Pink Demon.
The gear you acquire is Color-Coded for Your Convenience, based on the color the name of the item appears when you hover your mouse over it; uncommon items are referred to as "greens", rare are "blues", epics are "purples", and Legendary items are "oranges". Good luck on getting an orange, and no one beyond level ten or fifteen would willingly use a common or "white" item (except maybe hats or shoulder pieces, which are hard to find until around level 30).
When the Death Knight class was released in Wrath Of The Lich King, one of their spells was "Death Grip". What it basically did was yank an enemy player over to you so you could give him a face-full of your sword. In Cataclysm, priests got a new spell known as "Leap Of Faith", which did the exact same thing (yanking a player towards you to get them away from a big sword or area damage), only on friendly players. People thought "Leap Of Faith" was a stupid name, and so, unofficially renamed it "Life Grip". Even the devs use the nickname on occasion.
In Warlords of Draenor, the bosses Hans'gar and Franzok (a Dual Boss encounter) are popularly known as "Hans and Franz," not only because of the shortening of their names, but also because they are a clear reference to the recurring Saturday Night Live sketch, starring two Austrian bodybuilders, Hans and Franz (who were themselves pastiches of Arnold Schwarzenegger).
I Knew It: In the End Time instance, it is revealed that Murozond is Nozdormu's future self.
Promoted Fanboy: Several fans have been immortalized by Blizzard by making references to them, sometimes in the TCG as Alamo and Leeroy Jenkins, whose antics are legendary amongst the fans; and sometimes in the game itself like Maghia and Volde, who were two of the best cosplayers at Blizzcon 09, with items named after them. Notable Fansite creators as well — Breanni, the Pet Shop NPC in Dalaran, is based on the character of the go to site for Minipet info.
Beside Toskk, 3 other players got the same treatment. Rogue theorycrafter Aldriana, Warrior theorycrafter Landsoul, and creator of a program called "Rawr" that allows all classes to explore item options, Astrylian, all got drops named after them in the Icecrown Citadel 25 raid instance. Better yet, all of these except Landsoul's helm were the best items for their item slot for many classes up until the Cataclysm expansion.
10 year old boy Ezra "Ephoenix" Chatterton got quite a few references thanks to a Make-a-Wish foundation visit to Blizzard headquarters. He got an item named after him, and got to create a quest in the Tauren starting area. The quest even has you find a dog with the same name as his. The NPC has the rare honor of having unique voice acting, all clips done by Ezra, and surprisingly deep for a 10 year old boy. The unexpected questioning of "Can you help me find my dog?" is known to frightened many a player. Thankfully, Blizzard had the foresight to make the NPC unkillable by alliance players. Ezra passed away in October 2008.
The Lunar Festival NPC in Thunder Bluff has been renamed Elder Ezra Wheathoof, Wheathoof being the name of the above questgiver. He is accompanied by a Phoenix Hatchling.
Phaelia, who wrote a resto druid blog for a while before retiring to focus on the baby she was expecting, was also referenced with a leather healing chestpiece.
The Fact Checker even gets a cameo in the Dwarven Faction Leader short story, Fire and Iron. The story is about Kudran Wildhammer stepping down from his position on the Council of Three Hammers and giving it to Falstad. Some people have theorized the story was written in lieu of Red Shirt Guy's pointing out of the mistake, and has thus made Kudran's accidental promotion during the beta, as well as his replacement with Falstad, canon.
From the vanilla World of Warcraft manual, there were supposed to be dishonor points. The stuff certain players do on a common basis today in the game would have given them severe punishment (including being attacked on sight by their own faction's NPCs and even being exiled from their own cities) with this system if it were implemented past the open beta. By refraining from doing these actions for a long enough time, the stigma will eventually go away.
"Even among enemies as bitter as the Horde and Alliance, there is honor. If you flaunt this honor and engage in objectionable PvP play, such as killing new players vastly inferior to you in level, or killing essential non-combat NPC's such as flight masters or quest givers, you will earn dishonor. If you accumulate enough dishonor through your criminal actions, you will be branded an outlaw. As a consequence, you'll suffer experience penalties, lose access to your own faction cities, and become so hated even by your own kind that every faction NPC will attack you on sight."
Though the reason for this being taken out is ironically, prevent another form of griefing. When there were raids upon opposing cities low level players would be able to purposely get themselves killed from AoE attacks causing AoE users to rack up dishonor points.
The Undercity was going to be a lot bigger then what was given at game release; cutting out what would of been a balcony level above the bank in the large circular area of the Forsaken capital. The reason for the cut was because Blizzard thought that the Undercity was already complex enough, and that adding the balcony level would make it a little too complex for players. Even today after eight years, you can find and walk around the Undercity's incomplete second floor.
Azshara was a pretty but barren vanilla zone up until its Cataclysm revamp. Before then, Blizzard was planning to add a battleground called Azshara Crater at the southern edge of the zone to give players some reason to come to the area. The battleground was never implemented, but what would have been the entrances to it stayed in the game up until the Cataclysm revamp.
The Mage city of Dalaran was probably planned to play some eventual role in its original place that never manifested. Instead, there is now an empty crater where it used to be, with the city taking flight and acting as the central hub in Northrend.
The Duskwood zone was originally planned to have a light-radius effect on the player as you moved around in the world environment throughout this area; something you'd see out of the Diablo franchise, and torches were to be an item you could equip to increase the size of the light-radius. Even today, NPCs around Duskwood can be seen holding what would have been the player-held torches.
The ability for player characters to learn additional languages was planned early but was not developed, likely due to the tendency of cross-faction communication to be insulting and profane. It seems unlikely that this functionality will ever be added. The Forsaken were originally able to speak Common and thus communicate with Alliance players, but this was removed with the introduction of Gutterspeak due to, once again, the tendency of cross-faction communication to be insulting and profane.
Mages can now temporally learn the languages of their own faction with the Glyph of Arcane Language and the buff Arcane Brillance.
Azjol-Nerub was originally intended to be an entire underground zone, stretching across Northrend between the Dragonblight in the south and Icecrown Glacier in the north. Unfortunately, constraints that could not be overcome in the given product schedule forced this idea to be shelved, with some of the already created geometry folded into a pair of instance dungeons.
In a Blizzcon interview, Blizzard even admits that not turning Azjol-Nerub into a world zone was their greatest failure for the Wrath of the Lich King era.
Wrath of the Lich King was pretty bad in this regard overall. There are items in the game files that hinted that a third Utgarde Keep wing and the Gundrak temple were supposed to have had raids at one point in development; quest text in these areas seem to support this, especially in Zul'Drak.
Crystalsong Forest would have been the site of the Argent Tournament, but Blizzard was worried of possible lag carrying over from the nearby Dalaran hub. The area was left very barren of anything of interest.
The village of Unu'pe in Borean Tundra is a fully functioning quest hub lacking only the quests.
The Burning Crusade expansion was originally supposed to include so called "portal worlds"; portals that would lead to zones that would be on completely different planets.
The biggest one done so far: Path of the Titans. This was heavily advertised and showcased at Blizzcon before the open beta for Cataclysm to be a progression path past the level cap, but it was scrapped due to being too complicated. Instead, they chose to expand upon the glyph system introduced in Wrath.
Actually, it was scrapped because it would've just become another "If you're this class, pick this path for the best dps/heals/tanking". Glyphs and talents had already become cookie-cutter, so this would have just been another set of bonuses that you had no choice in. In expanding the glyph system, they added more room for choice (sort of).
Pandaren were originally planned to be the Alliance race during The Burning Crusade. Midway through development, presumably due to legal issues with China, the Pandaren were hastily replaced with the Draenei.
Cataclysm...oh boy, Cataclysm. Despite Blizzard's denials, it is commonly accepted that the Cataclysm development team suffered a cataclysm of their own; the end result: tons of cut features and storylines.
The aforementioned Path of the Titans, especially since it was tied so heavily into the Archaeology profession. End result: Archaeology was a boring and tedious grindfest dependent on RNG.
A proper 10-20 zone in Silverpine/Gilneas for Worgen players, with their district being in Stormwind instead of Darnassus. The quests after the battle to retake Gilneas City make little sense, and the Alliance version of Shadowfang Keep makes no sense.
The Abyssal Maw dungeon/raid, which despite developer comments that it never made it past the concept art stage, could still be found in the game files with some editing, as seen here. The cancellation of Abyssal Maw is a sore spot for players, especially since it caused a massive Aborted Arc the developers continue to sweep under the rug, and also because it was replaced with two troll raids converted into 5-mans.
A Silithus revamp, compete with quests that indicated that C'thun was active once again.
Boat races in Thousand Needles.
The War of the Ancients instance was originally supposed to be a raid, not a 5-man dungeon.
Foreshadowing for a questline involving the return of Danath Trollbane.
The Goblin racial leader is not in the game outside of the Goblin starting zone, despite having a voice set that would indicate that he is supposed to be residing in his titular Pleasure Palace.
Speaking of Goblins, the ending of their starting zone also ended up making little sense and may have originally had an alternate ending.
One small-yet-major change to the goblin starting zone was that, originally, the footbomb was supposed to fall into the volcano and destabilize it (in the final product, it was Deathwing that caused the issue, and the footbomb kick was instead an excuse to be looking in that direction). This would have given Gallywix's lines on the Lost Isles a completely different meaning; instead of him using you as a scapegoat (and becoming a Hate Sink as a result), he'd be reminding you of your part in the whole mess.
All references to Sylvanas mind controlling any newly-raised Forsaken were removed near the end of beta, but some of the quests in Silverpine Forest and Western Plaguelands still show mind controlled newly-raised Forsaken.
The Battle for Gilneas battleground was originally supposed to take place in Gilneas City, and the objectives were that each faction fought to see who could take the most districts of the city.
Warlords of Draenor, before even being released, had some scrapped ideas shared by the developers.
While Garrosh returning to make a new army had always been the plan, it was originally intended to be an army of all the NPC races players have been killing for years like kobolds, gnolls, centaur and troggs.
After that they considered having Garrosh be banished to Outland where he'd find a "magic horn" that reforms it into old Draenor and resurrects the Warlords from the dead. The developers thought this was too confusing and settled on the time travel version we have now.
The moving of the capital cities from well-known lore locations (Like the Black Temple) to generic outposts on the PvP hub. This particuar change has not sat well with players, doubly so because it was casually revealed on Twitter after the previous BlizzCon made a big fuss about it.
There are several missing or incomplete "hidden" levels including a copy of Undercity with no citizens, a creepy dungeon with drowned victims near Karazhan, an early incarnation of Outland with a set of portals that do not work, and a three-zone Emerald Dream area.
The Wiki Rule: WoWWiki came online one day after the game launched. Since then there was a schism, and now there is WoWWiki and Wowpedia.