People sure are spouting some wierd examples of what makes 25man content "harder". Frankly gathering 25 non-retards does not have anything to do with the content you are doing, and pre-tbc that number was 40 *shudders*. The true difficulty difference between 25 mans and 10mans is coordination. Simply put having more players in a fight means there is more going on that needs to be accounted for on a raid wide scale. And yes after we (my 10man guild) had kara on farm we pugged gruul's with another guild who was doing kara. Not one of us had ever done gruul's before but after 2 attempts on high king and 3 on gruul we had them both down in a night. We had the skill and the gear just needed the coordination and the understanding of the encounter. After that it was cake.
The difficulty, if it can even be called that, of a 25man raid isnt the fight itself its just not having too many retards in the raid, which has nothing to do with an individual players skill or gear or perfect understanding of an encounter. Its just that the more people you have the more likely you are to have enough retards to wipe everyone else.
10 skilled players wont have any harder a time than 25 skilled players in their content level would. As in skilled players, with the proper gear, understanding of the fight, awareness of their duties and communication with each other its all the same whether there are 10 or 25. People just say 25man is harder because there is nothing "harder" in the 10man range than ZA so there is really no apples to apples comparison. But with WotLK there will be which should prove quite interesting.