Personally, I think an optimistic Doctor is better than a pessimistic Doctor. I mean, he's been through so much that it's about time to take it all with a smile sometimes.
Also, I think his attitude towards humans could be based on this: "Sure humans are dumb but who WASN'T at their point? They're a lot better than the Vulcans were..."
It may have taken regeneration to realize that but I think that's what it is.
...Even if there aren't any Vulcans in Doctor Who...
But I do agree with how merciful he seems to be. I thought he'd gotten over that... But that's by no means anything new.
Just look at Genesis of the Daleks.