In Wrath, 10-mans are definitely easier than 25-mans. Not solely because you only have to get 10 people to learn all the fights, but in general the fights are just tuned to a lesser difficulty. For an anecdotal example, my guild recently killed Vezax on normal difficulty with a ranged DPS and a healer down, leaving one Paladin healer, a Balance Druid and enough melee DPS to cover interrupts. We had very little trouble, aside from the fact that Vezax started aiming Shadow Crashes and Marks of the Faceless at melee. In 25-man, minus half of the ranged DPS and half of the heals, most encounters would be impossible or at least very difficult.
As a result of the difference in difficulty between normal and heroic leaves me kind of confused. Blizzard has said that they want two separate progression paths, one that goes from heroics to 10-man Naxx to 10-man Ulduar (and presumably 10-man whatever comes next). And another that goes from heroics to 10-man Naxx to 25-man Naxx to 25-man Ulduar. If this is so, why are 10-mans easier? In BC this wasn’t the case. ZA was as hard or harder than Gruul and Magtheridon.
I, for one, love 10-mans. I find that they often draw out the people in a guild (that regularly does 25-man raiding) who really enjoy raiding and know their class particularly well. They also tend to be the more social players, from my experience. So I’d like Blizzard to give 10-mans a little bit more love, rather than making them toned down versions of the heroic instances. It’s hard to tell what exactly Blizzard’s plan is with 10-mans, whether they want to make them easier or equal difficulty. I think they managed this in vanilla, where Zul’Gurub was almost more difficult in certain places than Molten Core, the introductory 40-man.
So mostly I’m just confused. I enjoy doing 10-man Ulduar, hard modes or otherwise, but I’d like it to be clearer what role exactly the 10-mans play in the big picture of raiding.