Der Inhaltspatch v3.1 soll das nächste große Schlachtzugdungeon "Ulduar" (für zehn und 25 Spieler) einführen und Blizzard verspricht, dass der Schwierigkeitsgrad höher sein soll als bei Naxxramas, Malygos und Sartharion, die eher als Einstieg/Lernphase in das Schlachtzuggeschehen dienen.

Somit sollen alle Bosskämpfe herausfordernder sein und da die Entwickler sehr zufrieden mit Sartharion und seinen drei optionalen Drachen sind, wird es in Ulduar bei vielen Gefechten "hard modes" geben, wobei sich der Schlachtzug dann vor Ort entscheiden kann wie schwer der Kampf sein soll - außerdem ist ein optionaler, sehr schwerer Boss fest eingeplant. Als Belohnungen für "hard mode"-Gefechte winken diesmal nicht nur Achievements, Titel oder Reittiere, sondern auch bessere Beute, sofern man den Kampf erfolgreich meistert. Die Idee mit den "hard modes" will Blizzard in Zukunft weiter ausbauen.

Ulduar soll demnächst auf dem Testrealm verfügbar sein, aber längst nicht permanent wie Naxxramas bei der Lich King Beta. Zudem sollen während der Testphase nicht alle Bosse freigeschaltet sein, damit nur bestimmte Kämpfe getestet werden können.

Ulduar will be available on PTR in the future, however we are planning on having several limitations in place so that we can control what can be tested at any given time. Ulduar will also not be available permanently like Naxxramas was in the WotLK Beta. All this basically means that if there is something we want to test, then we will enable the zone and encounter so that it is available. If at some point there is something that we no longer want to test, then the zone will be disabled so that it is no longer available.

Keep in mind though, that there will be quite a few encounters to test in both the 10 and the 25 man versions, and there will also be a whole lot of "hard modes" and achievements to test. We are quite pleased with how Sartharion and his drakes worked, so we would like to expand on that idea quite a bit in Ulduar and future raids. We are currently planning for most of the encounters to have "hard modes" that will allow for raids to increase the difficulty. Increasing the challenge will of course also mean an increase of reward level if successful.

Ulduar will be on PTR in the future, but we're going to have tight controls in place on what can be tested at any given time. Ulduar will also not be up permanently like Naxxramas was in the WotLK Beta. If we have something to test, we'll make the zone and encounter available. If we don't, we'll disable the zone.

That being said, there's going to be a LOT of encounters to test in both 10 and 25 player modes. There will also be a whole lot of "hard modes" and achievements to test. We're shooting for most all of the encounters to have "hard modes" that allow raids to increase the challenge and reward level if they're up to it. We're pretty pleased with how Sartharion and his drakes worked, and we're going to expand on that idea extensively in Ulduar and future raids.

The reward for completing the hard modes are not going to be just achievement points, titles, or mounts, but improved, additional item rewards.

I think it's safe to say that Ulduar will be a bit more difficult overall than Naxxramas, even without the hard modes. We're still going to tune the zone for accessibility(*), but a raid group that's done Naxxramas, Malygos, and Sartharion and obtained the gear from those encounters should be well prepared for Ulduar. Not only is the raid's gear better, but players should now have some raiding experience under their belts. We also have a much more accurate picture of what guilds are capable of from a healing, damage, and tanking perspective then when WotLK first went live. If Naxxramas was the introduction to raiding 101 class for players who have never raided before, think of Ulduar as the second year class.
* There will be one optional encounter that is only tuned as a "hard mode" encounter.

I don't think you have to worry about spoilers from Achievements, to be honest. Seeing achievement text before you fight a boss is like seeing a single piece of a large jigsaw puzzle. Sure, it's something, but it's nothing really without seeing the context of the encounter as a whole. I also expect achievements to be something tested late in the PTR process. Implementing them is one of the last parts of the development pipeline (Quelle).