My guild cleared all of the 25-man raid content in Wrath within two weeks of the expansion's release. Naxxramas was easily the biggest non-surprise. Doing Naxx-25 in the company of people who know the place inside and out is a pretty straightforward and -- dare I say it -- easy process. This is even more true with players who learned the original Naxx at 70, with a much greater margin for error than they would have had at 60. Honestly? Most of the fights haven't changed to the point where you'd have to toss out your previous strategy and start all over again. A raid that saw Naxx at any point between 60 and 70 is effectively an old dog that doesn't have to learn a new trick.
Would guilds be breezing through content if Naxxramas had gone live at 80 without having appeared in the game before? I don't think so. Many of the fights are sufficiently complicated and/or difficult that guilds without any previous experience on them will be wiping to Heigan's dance, Patchwerk's damage, and Kel'thuzad's adds. It is not "gimme" content if you haven't seen it before, and I was reminded of this in a rather unsettling fashion after respeccing resto in order to heal a Naxx-10. I'd tanked a Naxx-25 with no issues, but I'd never healed a Naxx, and had to do so using mostly 70's level gear with feral glyphs still in (dual specs can't come fast enough). To say I got my ass kicked would be the understatement of the year. Trust me, Naxx is tuned just fine.
But was it a good idea to resurrect an elderly raid? I think so. It's thematically appropriate both to the tone and storyline of Wrath, and let's face it -- the most consistent complaint we saw of the level-60 Naxxramas was that almost nobody got to see it. Guilds who actually did it at 60 were universal in their praise for the design of the instance and the encounters; it was Blizzard's great experiment, and subsequent raids benefited from their experience planning and designing fights that revolved around better and more interesting mechanics in a more immersive environment. You can make a case for Black Temple and Hyjal being direct ideological descents of Naxxramas, which probably accounts for the sense of deja vu (or would that be jamais vu in the current Naxx?) you might get there if you saw Tier 6 content in BC.
Moreover, the two new raids -- Sartharion and Malygos -- have the notable distinction of being encounters with very little trash (or in Malygos' case, none at all). One of the heavier psychological tolls on progression raiders has always been having to reclear trash after wiping for ages on a boss. There is nothing more demoralizing than spending an hour clearing ugly, boring trash, only to spend 5-7 attempts getting thrashed by a progression boss, running back, rebuffing, and then hearing the groans on vent as trash begins to respawn. Say what you want about raiding nostalgia, but you'd be hard-pressed to find anyone who genuinely misses the old 45-minute respawn timer on Serpentshrine Cavern trash. And let's not forget that said 45-minute respawn timer arrived immediately after the raiding hell that was the transition from 40-man raids to 10-man pre-nerf Karazhan, and subsequently 25-man pre-nerf Gruul and Magtheridon. If you were one of the people who did these encounters when they were first released, then it's certainly a feather in your cap to have done them when they were extremely difficult -- but let's be honest, you don't miss those days.
So out of current Wrath raid content, we have a primary raid that's been in the game for almost 2 1/2 years (Naxx went live with patch 1.11 on June 20th, 2006), and two single-boss encounters with little to no trash. I would have been amazed if raid progression hadn't been as fast as it was, and I'm pretty sure that that was Blizzard's expectation as well. They've said they want things to be more about fun and accessibility, and less about pointless slogs. Nobody wants the game to feel like another job.
If we accept that early raiding content was essentially meant to be this fast for hardcore raiders, that Naxx was "recycled" for a good reason, and that it's beneficial if early raids aren't as hellish as BC's initial content, the question really isn't whether the early raid content's too easy. I think the real question is whether more raid content should have gone live with Wrath's release in order to keep the hardcore guilds occupied until the next content patch, and whether Blizzard intends to keep the more advanced raid content (e.g. Ulduar and Icecrown Citadel) "easy."
The answer to the first one, at least from my perspective, is a solid /shrug. There's a ton of stuff to do in Wrath that has nothing to do with raiding, and if you can't keep yourself thoroughly occupied until the next content patch, then you're probably not looking very hard. Otherwise, you're not really playing the game for the sake of playing World of Warcraft: Wrath of the Lich King; what you're really playing is the larger meta-game of beating players worldwide to the completion of new content.
The answer to the second remains to be seen, but I don't think Uldar and Icecrown are going to be easy at all. By the time they go live, Blizzard will have every right to expect hardcore raiders to be thoroughly geared, and no one will have the benefit of previous experience or months of fiddling on the beta to inform raid strategies. And, well, the pessimist in me also expects to see lots and lots of trash.
People worry about the ease of raiding because a huge part of what makes raiding fun is feeling like you earned what you got. If it's too easy, then the cool gear on your toon doesn't actually mean much because you didn't expend much effort to get it. Too hard, and the ratio of work to reward starts to feel somewhat ridiculous for what is, after all, a game. It's not the easiest balance in the world, but also not one that I think Blizzard (or players) should worry about in the first tier of raid content in Wrath. If Ensidia one-shots the 25-man Arthas on the test realms, that would be a serious problem -- but for the moment, I'm not going to lose sleep over the accessibility of content that most players have traditionally never had a chance to see.