Quickshiv:

One thing you and a lot of other people are not reallizing is that it is really hard to make a well-tuned 5 or 10 man *hard* without making it *impossible* without the correct group makeup. To give a a few examples, Magister's Terrace was a pretty difficult heroic. Generally speaking, it took certain group compositions, ie sufficient cc. When they're designing 5-mans, they have to consider all the different classes and possible group makeups, and cannot assume that any particular class or ability will be present, other than tank, heals, and dps. That is, you have limits to your design because you can't assume the presence of two tanks or two healers.

In 10-mans, they have some more freedom. They can require 2 or 3 tanks and healers by boss mechanics, giving a lot more freedom in encounter design, both in variety and difficulty. Still though, they can't assume every buff is present (ie bloodlust) and have to tune the encounters accordingly. Take note of the obedience crystals on Instructor, making this fight easier than the 25-man, or the MC that KT does. Generally speaking 10-mans have to be tuned more loosely than 25-mans.

In a 25-man, they tune encounters assuming every single buff will be available and used. Whereas in 10-man they don't want to punish any class or spec for not bringing enough buffs, in 25-man they can assume every class, buff, and ability is available.

Now, obviously, this isn't true in Naxx. As someone else points out later on, Blizzard basically admitted that they messed up on tuning. 10-man is about right, but the 25-mans (at least Naxx) were made too easy--GC has said so multiple times. The point is, they *can* make 25-mans harder, but failed in Naxx.

That said... I wonder if there really isn't something to 25-mans being somewhat harder, based on all the responses here, many of which boil down to, "Why would players want to do more work to get the same gear?"