That is something I'm thankful for, that they didn't opt for another "Matt Smith" Doctor because of his popularity with the younger demographic. In picking an older, different, Doctor, it says to me that they have maintained a certain integrity in picking the right Doctor, and not particularity picking a popular "type" to play him.
I also think the new Doctor will be the right match for River, this might be the Doctor that River has true chemistry with. River and Matt's Doctor never seemed to mesh, I never believed the Matt's Doctor ever really cared as much for River as she did for him. Matt's Doctor, Amy, and Rory all seemed to be contemporaries, while River always felt like the "old man out". They couldn't related to her, even given her obvious connection to all of them, in my mind, she never fit in with them. This new Doctor may give us the true romance between them that makes her final appearance make more sense. He could be the more mature Doctor that helps River "grow up" a bit, they will grow together, and she'll become the more grounded Professor we saw in the Library.