Dungeon Finder
The most revolutionary feature since the expansion would have to be the Dungeon Finder system added in Patch 3.3.0. Using it is completely effortless. You click what you want to do, the system automatically throws a viable group together to do it. In most cases you wait less than 20 minutes, during which you can continue questing/farming/auctioning, then when the group is made you are whisked to your dungeon.
The flaws, as usual for Blizzard, are few. Bad players can still ruin the experience, but it is fairly easy in my experience to replace most of them with the Vote to Kick option. The queue to join, however, is completely driven by the shortfall of tanks (and sometimes healers) using the system. Due to this, you can usually get in instantly if you play these support roles. One of my tank alts hasn't moved away from the auction house in 12 levels, since there's no need to quest if I don't want to. There were some issues with loot fairness, but Blizzard is acting quickly to seal the gaps.
Ulduar
After the initial set of raid dungeons came the addition of Ulduar. A huge, sprawling, multi-themed work of art, Ulduar featured 13 bosses and a semi-secret ultimate boss for the very elite. I think the most creative fights currently in the game are all in Ulduar. Let's talk about a few.
The first encounter, Flame Leviathan, is a vehicle-based gauntlet clear followed by a vehicle boss. You are given a set number of 3 different vehicles, each with 2 seats, that must work in concert to earn victory. For example, the Demolisher's driver is responsible for dealing the bulk of the damage, but the player in his gunner chair is responsible for shooting down from the sky and collecting crates of the ammo he needs. The gunner can also be catapulted onto the Flame Leviathan to disable the boss for a short time.
This fight is the first example of a variable difficulty encounter. Up to Four different towers can be left standing during the gauntlet approach to the fight, each making the fight more difficult in a new way. One will shoot a trail of deadly fire into the playfield. Another spawns an unholy number of monster plants you must deal with. Each tower left up increases the rewards you get.
The last (non-secret) encounter, Yogg Saron, is completely insane. The flavor revolves around it, in fact. Yogg and his minions do their best to destroy your 'Sanity' (a special counter that appears at the start). If they succeed, you turn on your friends and cannot be saved. However, you don't actually engage Yogg himself directly until the very end of the fight. His mind tricks manifest in several ways, forcing you to dance around a circular frogger-style pattern of moving clouds. Then he summons several different types of tentacles, all with various treatments, all while your melee players should be teleporting to hallucinations of Azeroth's past... It's too crazy to explain, but very, very, cool. This fight also has 5 difficulty settings. 2 dungeons later, with far better gear, my guild has yet to beat the last 2, and my guild is pretty good. The very elite could win before the extra gear, which says a lot about how great the best players are, and how well Blizzard pushes the limit for these few.
Trial of the Crusader
A shorter, one-room raid, Trial has 5 bosses, and only 2 modes each. I'm bummed that they seem to be moving away from the Ulduar style of multi-stage hard modes, which you engage by behaving differently. Now you just flip a switch in the UI. Less cool, to me.
I enjoyed all the fights in Trial, but there's nothing too fantastic or new here. Even the boss models are recycled from other content. That combined with the tiny loot table left my guild feeling burnt on it quickly. I was pleased that we mastered the hard modes before new content came out (unlike with Ulduar), but I'm betting that means better guilds were even more bored.
One fight of note is another 'fake-PvP' encounter, Faction Champions. Your party of 10 faces 6 NPC enemies, which behave similar to players in PvP. They use CC, dispel and heal each other, seem to pick on the weak characters together, and generally murder your face with scary unpredictable stuff. They follow the player rules for CC duration and a special AOE damage reduction rule. I'm mostly impressed by the realistic appearance of the NPCs, even though they cheat and have 20x your health, they feel pretty smart. I'm guessing there's not really a lot to them other than a bigger ability list than usual, and some random target switching code. MMO PvP bots might be easier to make than I thought.
Icecrown Citadel
The end of this expansion's story, you face Arthas and his army on the way up his multi-story ice fortress. Icecrown Citadel feels very big, but the theme seems less fantastic to me than Ulduar. There is less color variety, and really very few notable set pieces. I was somewhat let down after the build up that was Icecrown the overworld zone. I expected a bit more phasing play. Involvement outside with the armies of the Alliance was what I expected. Instead, the King (Varian Wrynn, not Arthas) just stands at the doorway giving you encouraging words.
Some of IC's fights are very good. Mostly, they are very hard. I'm okay with that, personally, but Blizzard has obsoleted older content through aggressive badge upgrading, so there's not much to do other than bang on whatever fights your guild can do here every week until new stuff comes out. I expect quick guild burnout and lots of attendance problems for us and many other guilds until the next expansion.
The fight against Arthas is notable in difficulty. Tanking takes a step up, with several phases that require very active performance from one or both tanks. I haven't tried healing or dps this fight, but both appear to be just as dire. A few nasty movement challenges may put this fight out of reach forever for any guilds with a few keyboard turners. In the end, it's very exciting to win. Killing a WoW icon is definitely satisfying. My only complaint is the lore that could have been. Where's Jaina? Sylvanas? King Wrynn or Thrall?