Ulduar is, in every way, a step up from Naxxramas. Of course, your gear has to be much better than what you could venture into Naxxramas. It is designed for full naxx10 gear, but having bits from the 25-man instance helps a lot. The fights are now much more complex, and in my opinion, more unforgiving. I remember people complaining about the nuances of defeating the Four Horsemen or Kel'Thuzad, but I think it is safe to say, Ulduar takes it all much further.
Looking at the first four bosses, they are by far the simplest on the complexity scale, on par with the toughest in Naxxramas, but if you are geared and know the tactics, they should not constitute a huge problem. In fact, the same way that arachnid- and plague quarters in Naxxramas, the first four of Ulduar can easily become the farm bosses for most guilds that venture in.
From there, the fights quickly begin to get tougher. From the antechamber, both Kologarn and Hodir were quite fun for me. True, the new guys (me being one of them) struggled a bit on Hodir but after a couple of takes, dodging the flash freeze and falling rock became quite fun. The one annoying boss, was Auriaya and her two cats. I am by nature a tank, even if I spend most of my time as DPS, and as such I like fights that I can control. If there is chaos on the battlefield, it puts me off and the two cats are an epitome of chaos. The quicker you take them down, the better you will be off.
When you get to the keepers, you are truly inside Ulduar. Tougher than anything you have seen in WotLK, by far (not counting Sartharion with three drakes). We took down Thorim fairly easily, thanks to the fact that we had some experienced people on board but both Mimiron and especially Freya, were a bane. Both bosses share the fact that there are a lot of things to keep track of on the screen and it takes quite a lot out of you, whether it is avoiding mines, dodging shotgun shots, looking for the healing tree, or coordinating to kill lashers. And in middle of all that, you should also do some decent dps.
It is difficult to explain how hard we tried to take Freya down. We did it eventually, with a near flawless performance in the end but I have to say, it was an undertaking. People say that many of the fights in Ulduar are strictly a gear check, like XT-002. While that might be true, Freya on the other hand, is a check on team work. Everyone has to pitch in; to kill the trees, aoe the plants, kill the three children at the same time, and free rooted compared. After my experience, I am a firm believer that until your group can really work as a team, you will fail on Freya.
From there, the next two bosses are naturally only General Vezax and the old god himself, Yogg-Saron. I could not make the last raid so I was only there for two attempts on Vezax but the fights do continue quite complicated. For me, as a death knight, Vezax is very easy because I am unaffected by his most frustrating ability; Aura of Despair which prevents almost all mana regeneration. Healers especially however are at the receiving end of that fight though, without a doubt always running out of mana and having to dodge in and out of green puddles.
Yogg-Saron will be an interesting fight and I cannot wait to get a chance to face him on next lockdown. Research has promised an interesting battle of wills; as another fight with multiple phases and many small nuances. As a whole, I have come to enjoy Ulduar very much. The fights are interesting and very colorful. There are a myriad of challenges and there is much to look forward to, and not not in only killing Yogg-Saron but gearing up for the hard modes and beginning a spree for a bucket load of achievements.