Blackrock Depths is one of the biggest instances in WoW by just about any measure. It has a wide level range, from 48 to 60. In terms of bosses, I count 20 encounters. Think about that, 20 bosses in one instance. I'll come back to the bosses near the end.
The actual floor space of it is huge, comparable to some raids. I've heard rumors that it was originally going to be a raid, continuous with Molten Core; though the first I ever saw it it was capped at 10 players with some quests I believe doable only with 5. BRD is not flat either. It has tunnels which twist and turn and go over and under and all around. There is a river of lava which goes all around the instance, which old-timers will give some special importance (I'll explain it).
BRD isn't just big though. Oh no. It is also complex. It is a city with all the complexity of a city. It has the bank and bar and senate and peasants and I've said this before...
Here's what truly makes BRD unique: it reacts to your actions. This is more than just press button to open door. Oh no, this is about choices and it actually mattering what you choose to do.
Most instances you just kill mobs. Pull, kill. Maybe you go right instead of left or pull an extra group, but in the end you're mostly just killing. BRD has plenty of killing, but also much more. I'll go through these, but it may end up being a little bit spoilerish. Skip the bar and bank sections if you want to discover things on your own, come back if you're not sure if you found it all.
You must die and talk to a ghost to get the starting quest. Once you have the key, it starts to open up choices. You can go through certain locked doors to skip mobs. The biggest effect though is being able to close one of the massive gates along the Dark Iron Highway. Why would you want to block your progress? There's a boss behind there! And another one behind there, and another, and so on. Closing the gate also connects a tunnel through the top of it, allowing access to new parts of the city.
The parts you gain access to do not include the final boss. He was always accessible, if you didn't mind swimming in some lava. Yes, you can get to the end by swimming in lava, a lot of lava.
The room before him has a name I forgot and some giant like those in Molten Core. He's boring and who cares? The room before that is called the Lycaem. This is a huge room, filled with non-elites. Ugh, mindless trash. Nope. They also respawn really fast, so you have to kill and move quickly. You don't just have to cross the room though. You must also find and kill a couple special mobs carrying torches in order to light the fires that open the doors at the other end. This room was very difficult at level 60 and I've never seen a similar mechanic every since.
The emperor himself is surrounded by the Dark Iron Senate. He yells at you as you kill them, though if you pull him first it gets more interesting. Oh yea, and the dwarf with him, that's the kidnapped, brainwashed daughter of the current dwarf king. Do you really want to make her a single mother?
The bar is probably the best part of BRD for ths sheer number of options it holds. There are several bosses in there, but good luck figuring out how to fight them.
One is from breaking kegs, another you must threaten for being a bad guy. The third refuses to talk to you. What will wake up Phalanx? The last is the bartender, but he won't fight you either, he just wants to sell you booze and strangely, alchemy recipes. Oh, and you need to get through that locked door. What do you do now?
You have options. A rogue could pickpocket the key, but the bartender will notice and he won't be eager to sell you anymore alcohol. Try the usual violent route, attacking the patrons, and eventually guards will show up, opening the door while also guarding it. Two options and one not even violent? That's only half! Talk to the succubus and help her make a love potion; after that she'll gladly open the door, in order to seduce one of the patrons out back. Finally there's the semi-violent route: get one particular dwarf so drunk that he causes trouble; the property damage will wake up Phalanx, and also destroy the door in the process.
Find another instance which has four ways to get past an obstacle. And if you're willing to skip the bosses in between, you can get around it by going straight to the Lycaem with some lava swimming, so that's effectively five.
Oh, and there's a band. And a mole digger straight to the bar. These might be related.
Yes, the original arena. No PvP in here though. It's a little bit like the events in Nagrand and Zul'Drak, fighting off a powerful enemy, except rather than a contest for gold it's the method of execution (you've committed crimes against the Dark Iron nation). The bosses are random, dropping a mail physical damage set which was quite good for shamans and even hunters back in the day, along with random other loot. Note that the fans above are eager to see you die, at least until you've proven how awesome you are.
The arena is also used for what was at the time, a challenging quest event. As part of the dungeon 2 upgrade chain you did an extra fight in there. It was a chaotic sort of fight, perhaps a preview of the third boss of Magister's Terrace.
Open the picture and read it, I dare you. That triggers a randomly spaced boss, though the picture gives a strong hint as to where he is, which might be walking towards you. All those vaults can be opened with keys looted off the Dark Iron dwarves in the instance, though opening too many might alert the guards. There are two secret vaults as well if you can figure out how to get to them.
Ina typical instance, who are the bosses? For the most part they are best described as "no one knows and no one cares." They have no real story or personality. They're loot bags. BRD bosses are different. Okay fine, a few seem to just be generic "this thing is in your path and it has loot" sort of things. Maybe half are like that. But then there's the angry bartender, who would have sold you drinks only moments before, except he had his pockets picked and then was attacked. There's the golem who is the stern but not hostile bouncer, until someone causes trouble. There's the evil crafter of equally evil golems, who you know to kill because you are psychic (you'll get it if you do the quests for BRD), but who also isn't what he seems. There's the arrogant emperor, only too happy to watch you eliminate the legislature. The seven dwarves, who are not nearly so kind as in the story, but will you challenge them in order to break their bonds? How about the terrible architect who seized the claim to chief architect after the greatest designer among the Dark Irons was killed? These are not mere bosses. These are characters you'd expect to see. That they carry loot is a surprise rather than an expectation, for they are something other than mere bags of loot. Where else will you find so many NPCs who have a story and reason to be there?
That's all folks!
Probably not. You think you've done a full clear? You probably missed a boss. Try going down that tunnel. That one over there too. Did you get that area? Did you talk to him? 20 boss encounters, count em! Oh, and next time you come through, try a different path.
If you do manage to run out of things to explore, head upward to Blackrock Spire. Yea, the entire mountain was carved out by dwarves, that's why it all looks like that. Watch out for the dragons, and be careful about wearing a dress...