Daelo, the lead encounter designer, has dropped a few more details about Ulduar, the tier 8 raid that is coming in patch 3.1 (whenever that happens to be). We heard yesterday that there will be more "hard modes" in the raid, analogous to Sartharion. Today we learn that, again like Sartharion, completing the hard modes will give you not only achievements but also additional, better loot. There is also, apparently, going to be an optional encounter in the raid that will be hard-mode-only; I'm picturing something like Nightbane here.
Furthermore, Ulduar in general, according to the post, will be harder than Naxxramas is. Naxx was tuned easy on purpose, and the devs have been saying for a while that Ulduar will not be that way, to the same extent. The analogy Daelo makes in his post is that if Naxx is "the introduction to raiding 101 class for players who have never raided before," Ulduar is "the second year class." So we won't be hitting a brick wall, but it probably won't be compared to UBRS either like Naxx often is now. Either way, we won't know much for sure until it's up on the PTR, but I'm certainly looking forward to it.