1. For example most people claim that Ultraxion is fundamentally harder in 25man. I believe they are right. Though to be honest I disagree for their reasons. It's not harder because of numbers, numbers are probably thinner for 10man (because of the restrictions in buffing and indirect buffing) but it's a boss that is simple but does not forgive mistakes from every single person in the fight. Hence with 25man people the frequency of mistakes is much higher (or at least the frequency of inefficient gameplay).
2. A question: Is Zon'Ozz or Hagara harder in 10man?