When Wrath was released, there were just over a dozen bosses available to fight immediately. Most of them were found in Naxxramas, which fell to the superguilds so quickly that not all of them were even level 80 yet. A few achievements like A Poke in the Eye and the first "hard mode" of Sartharion with three drakes were all that the talented raiders had to keep them company during the first few months of Northrend. By the time that Ulduar was released, nearly everyone was clamoring for new content.
Cataclysm's launch hasn't shared the same fate. With heroic raids now the de facto form of progression and miles of new non-raiding content to explore, the content available at release has definitely lasted far longer. Even the most difficult bosses are still not on farm for the big guilds. Blizzard's developers have used this opportunity to start working on the first major patch after Cataclysm's release, which is of course includes plenty of holy paladin changes to keep us on our toes.