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.