I think you need certain qualities to be a successful PM, that are not just the obvious ones like charisma or good communication skills.
Most important is a kind of intelligence that's not necessarily book smart - if academic cleverness was the main thing, Enoch Powell would have been PM - but something I think of as political intelligence, by analogy with emotional intelligence. Blair was unusually good at this - people who worked for him say that he might not grasp the technical detail of an issue, but he'd get the politics of it very quickly.
Then I'd say a vision for what you want to do with power, where you can have some flexibility but you're obviously setting a direction for the government.
And also, because the PM has to do enormous amounts of bureaucratic paperwork, a high boredom threshold.
Because these qualities aren't very common in our political class, I think the parties latch onto rising stars who seem to have bits of these qualities, and it's not until they become leader that we see what they're lacking.
And that's even without thinking of policy. If there's one minister in this government who knows what he wants to do and is good at using the machinery of state to get it done, that's Ed Miliband. Whether you think what he's doing is a good thing... that's another matter.