Although not a proper zone, with quest hubs, flight plans and NPC's, Blackrock Mountain was a major part of the game back when Level 60 was the cap, and now that Cataclysm has been released, the developers have added some new content to this area which has made it popular once again. Although not all of the content featured inside the mountain is appropriate for Level 52, it will be applicable as I progress, so I wanted to put this here as a point of reference.
At launch, Blackrock Mountain contained three dungeons, Blackrock Depths and Lower & Upper Blackrock Spire. It also housed the game's first available raid, Molten Core, which required 40 players to clear. Eventually, a second raid, Blackwing Lair, was added, which meant that raiding guilds would spend most of their time in the mountain, occasionally shooting over to Kalimdor to take on the one-boss Onyxia raid. Because of its popularity, Blackrock Mountain became a heavily-contested area on PvP servers, with both factions stirring up trouble near the dungeon and raid entrances.
Inside the mountain, a winding stone path circles a deep pit of lava, which is the heart of the volcano. A tall stone pillar juts out of the lava, and is accessible via massive chains, which are your mode of transport into the depths of the mountain.
Down the chain and past a winding path, you encounter a mine where slaves are laboring to find precious stones and ore. Located in the back of the mine is the entrance to the Dark Iron stronghold, Blackrock Depths.
Blackrock Depths also houses the entrance to Molten Core, literally the volcanic center of Blackrock Mountain, but you need to be Level 60 to enter. Also, an elf who is further up the chain inside the mountain will allow you to become 'attuned' to enter Molten Core, so that every time you want to go in you don't need to go through all of Blackrock Depths.
Back at the top of the chain, a player can also choose to ascend the mountain by taking a series of paths that lead to the entrance of Blackrock Spire. This dungeon, which is actually split between Lower and Upper, is under the control of the Black Dragonflight, led by Nefarian, the son of Deathwing, who here is in disguise as a human. Like Blackrock Depths containing the entrance to Molten Core, Upper Blackrock Spire houses the entrance to Blackwing Lair, but there is an attunement quest available outside.