Ulduar was released in April, 2009. The first new raid released in 2009, Ulduar seemed to reflect all of the lessons, design paradigms, and feedback responses garnered from its predecessors. The raid proved to be a delight to folks looking for more exclusivity and challenge in their raiding, but also a frustration to folks who hadn't had much trouble with Naxxramas.
Ulduar technically housed a mere 14 bosses, which might have made it feel smaller than Naxxramas. However, Ulduar had something that Naxxramas didn't really have: challenging trash mobs. While each fight in Ulduar felt like an exploration of new design philosophy, the bosses were separated by trash mobs which were nearly as difficult as any boss in Naxxramas. In the end, that trash was so difficult they got nerfed over time.
Flame Leviathan, of course, was the first fight in Ulduar. One of the (thankfully) few vehicle fights in Wrath, Flame Leviathan did right everything that went wrong with Malygos's third phase. The combat was restricted to two dimensions of player movement. While gunners could (and should) fire up into the air, you didn't have to keep track of your location and movement in terms of up and down. The vehicles scaled with your gear, which meant players started keeping track of "highest item level" profiles in their gear managers. Even more importantly, the vehicle fight moved at an even, understandable pace. If the Malygos dragon ride felt like a frantic rush, Flame Leviathan and its trash were a slow, steady joy ride.
Immediately following Flame Leviathan, of course, was where players encountered that difficult trash. If you were going to fight Ignis next, then you had to contend with Molten Colossus.. And then you fought Magma Ragers. Each of these trash mobs had tactics and challenges as complex as Noth the Plaguebringer. You had to LOS your healers away from the giants, so that their heals and casts weren't interrupted. You had to carefully monitor which raid member was the target of the flame whirlwind, or risk devastation wrecking your raid.
Bosses like XT-Deconstructor exemplified the Hard Mode philosophy we previously saw with Sartharion. By killing XT's heart in a single phase, you turned the manageable robot boss into a raid-wrecking machine. But if you managed to down him in this hard mode, the gear was much, much better. Not every boss enjoyed this dynamic. Razorscale, for example, is basically an add fight and a tank 'n' spank no matter how you do it.
Every boss in Ulduar had a dance or new trick involved. It felt like the developers were responding to forum feedback that Naxxramas was too easy, that there was no challenge left in the game. While most of the fights in Ulduar were actually optional (no one made you fight Razorscale, you could just walk by him), any raid trying to gear up really needed to handle every encounter.
For all of the difficulty and new challenges in this new raid instance, a subtle benefit has come to light ever since. In my opinion, no other raid instance has so firmly captured the imagination of machinima creators. That might not seem like a big deal, but I think it goes a long way toward understanding why Ulduar was such a popular instance.
The lore of Ulduar was rich, engaging, and could actually be traced all the way back to old world content. For example, remember Uldaman? Uldaman was created by the same fellows who created Ulduar. The Discs of Norgannon aren't directly linked to the whys-and-wherefores of Ulduar, but the genre and story lines draw a connection to the Titans.
Yogg-Saron's Lovecraftian story is completely saturated throughout Northrend, but raiders finally get a crack at the old god in Ulduar. The fight against Yogg-Saron is rich, beautiful lore that you would miss if you didn't get to see it. That's why the accessibility doctrine in 2009 is so important: any player engaged in the story and experience of World of Warcraft would suffer a disservice without a fair opportunity to see that fight. While machinima videos do a good job of conveying that experience, there's nothing quite like seeing it yourself.
Finally, Ulduar housed a fight which provides a direct link to the Titans who created Azeroth. Algalon is the true final boss of Ulduar. Having gauged Azeroth too corrupt to live, Algalon is interrupted in his communication back to the Titans by your raid stomping its way into his chambers. Your raiders are once again in the position to save the world, even if there are hints that maybe this story isn't done.
In my opinion, Ulduar was the raid of 2009. Ulduar was a definitive experience in both lore, fight design, and variable Hard Modes that let raiders choose their own difficulty. If you weren't trying to do bleeding-edge challenges, you could blow through on normal difficulty. Even in normal mode, though, some of the fights required new skills and better coordination. If you wanted something more difficult, however, you could always press Mimiron's Big Red Button and take your raid up to a new level.