We're all familiar with the strange case of Azjol-Nerub. Originally it was intended to be a complete underground zone that players were to level in, do quests, and so on. Then that was scrapped and we instead got two instances and an NPC who was clearly the remnant of a once far more expansive plot. It's a shame, too, because those two instances are tantalizing hints at how vast and expansive an Azjol-Nerub zone could have been. Frankly, I found (and still find) the Nerubian architecture in those instances far superior to Naxx both in design and its surprising color palette. And when you look around Ahn-Katet and realize how vast the cavern is and how little of it is actually seen in the instance, or run Trial of the Crusader and fall into yet another astonishingly vast and underused space, I don't see how you can not wonder why the Azjol-Nerub zone never manifested itself. Perhaps we'll see World of Warcraft: Underground at some point.
But while in Azjol-Nerub we have a zone that failed to materialize, in Crystalsong Forest we have a fully developed zone with floating, magically altered forests of shattered crystal trees filled with ruins and NPC's and... no real point. There's two flight points, one for each faction, but you could easily miss them because why would you go to them with Dalaran floating right there overhead anyway and no quests to drive you down to them? Why does each faction need its own flight point in the region? Are they fighting over some objective? What is it?
Likewise, Dalaran's presence is intended to somehow oppose the Blue Dragonflight but aside from erecting yet another giant purple bubble (man, they love giant purple bubbles) they don't really seem to be doing anything to all those Blue Dragonflight mobs floating around on flying platforms in the zone. You get sent to Crystalsong by the Argent Crusade a couple of times, first off when they're pushing into Icecrown and then after they've erected their tournament and want you to kill satyrs for scrying crystals or knock out the Black Knight's squire.
This just begs the question: why is the Black Knight's squire in Crystalsong? Why are the satyrs there? There are ancient night elf ghosts haunting the ruins, the Blue Dragonflight, this zone should be positively bustling with player activity and instead it's a ghost town because there's no actual content here, just hints at it. There's a really interesting bit of backstory hinting at a war between the Black and Blue Dragonflights here that made the trees turn to crystal, and the Lich King's attempt to harvest the forest's power, but it doesn't actually appear anywhere in the game that I've seen. Crystalsong is basically just a very elaborate backdrop to fly over to get to Dalaran at this point.
Another thing that's interesting when looking through the data files is how many items that ended up in Naxx and (more recently) the new five man instances were, at least during the design process, intended to drop somewhere else. For instance, here you can see various familiar shield designs but also at the top and bottom you'll notice two designs you've yet to see. One of them, the bottom design, has an art style that clearly matches the Hate-Forged Cleaver which drops in regular Halls of Reflection, and which you can find in the item files as 'axe_1h_draktharon_d_01'.
Several sword models, as an example, either appear only on NPC's or in instances completely disconnected from their file names. Here, for example, we see several swords named for the Nexus, but only one of them drops in the Eye of Eternity raid, the rest all drop in Naxxramas (or Forge of Souls in the case of one of the reskins). Again, this isn't terribly unusual: several of the original sword models in the first release of World of Warcraft have names that hint at a faction bias that never came to pass, for example. We can, however, speculate that perhaps there was to originally have been more expansive content at Utgarde Keep, Drak'Tharon/Gundrak and the Nexus than actually came to pass, and that several of the art assets and models originally designed for these zones ended up moving to other instances like Naxxramas 25. (Naxx 10 more or less just re-used original Naxx 40 models.)
Now, at this point I hasten to add I am just speculating. I have no way of knowing and have found no conclusive comments from Blizzard saying "Yeah, we were going to do a bunch more raids but then we didn't and we just focused on Naxx instead" but it is clear that there was a great deal of work put into designing item models for zones that they never appeared in or appeared only on NPC's such as Ulduar. (And yes, I still wish that big Blinkstrike looking 2h had dropped somewhere, but I'm glad to see that axe and 2h sword get into player hands.) It's hard not to get caught up in wondering what Wrath would have looked like with more raids at start... one can imagine an Utgarde Keep with a UP raid on top. The place certainly seems big enough for a raid with King Ymiron having gotten a lot of build up in Howling Fjord only to then sort of fizzle out by being farmed constantly by pre-Naxx guilds looking for a Red Sword of Courage.
Likewise, both Drak'Tharon Keep and Gun'Drak just look overdesigned for the amount of content accessible: there's multiple structures and portals at Gun'Drak. Not only does Drak'Tharon look like you could easily have another instance there, the one we have is kind of confused and unfinished to me. Who exactly is the Prophet Tharon'ja? He looks vaguely like the Avatar of Hakkar from Sunken Temple... is he intended to be the harbinger for the various priests of the animal gods we encounter cannibalizing their own gods in Zul'Drak? Because we already have one of those, and he actually looks identical to Tharon'ja until you drain him for Quetz'lun's spirit. I'm fascinated by the lore hints in all the Zul'Drak instances (heck, the animal gods from Zul'Aman show up! Thankfully they didn't hold a grudge) and so, the idea that there was a possible raid exploring this in more depth and maybe explaining what exactly gave them the idea of eating their gods in the first place would have been fine by me. Maybe we could even find out what that enormous serpent tail in Gundrak is connected to.
I have no idea why, if there was work on UP and Drak'Tharon raids, that work was stopped. Then again, I don't know why Azjol-Nerub didn't pan out as an underground zone or why Crystalsong just sits there like an endless footnote in the Nexus war. I don't even know why we kill Malygos, Aspect of Magic and lord of the Blue Dragonflight, two raid tiers before we fight his undead consort. An argument could be made for Malygos as an immediate and arbitrary threat that needed to be dealt with, of course. It's just always seemed odd to me that we killed Malygos, empowered by Norgannon himself as Aspect, and then we go to the Titan-constructed prison of Ulduar and there's no real connection between the two events. It sometimes feels that in their drive to put Arthas on center stage they took the spotlight away from others too soon, and left a lot of interesting ideas undeveloped.