Wrath is almost all wrapped up, and while we didn't know much about it before the expansion, we've all certainly experienced the token system that Blizzard implemented as they went along, where early instances drop one kind of badge/token, and then the newer instances offer up new tokens, which can then be exchanged back for the older ones and their rewards. Now that we see the big picture at the end of the expansion, it's pretty ingenious, actually, and it even allows Blizzard to beef up other parts of the game, as they did with the rewards in the new Dungeon system.
Not that he needs to, but Bornakk steps up on the forums to justify exactly this kind of tiered system. Players complain that Ulduar is "useless" now that you can obtain its badges from lots of different places, but Bornakk says this system is definitely preferable to what Blizzard did in vanilla and BC, which was requiring new raiders to run through all of the old content before seeing the new and shiny stuff. They don't want the old content to sit useless (and it's not -- lots of guilds are still running Ulduar and even Naxx for the hard modes and achievements), but after the high-end raiders have their fun, it's important to get everyone else up to speed as well.