Still desperately hope this thing doesn't turn out to be the recast that we've all been told to expect, but I am going to leap to the defence of Pegg being cast in a serious role.
Comedic actors make the jump all the time, even in Star Trek. Remember Christoper Lloyd as the Klingon Commander in Star Trek III? When I see him in Star Trek III, I never think of Doc Brown from Back to the Future. Part of that, I grant, is the make-up... but still.