He's got far too little ministerial experience. Despite being Brown's adviser at the Treasury for most (all?) of his tenure, he's only been an MP since 2005 and a junior minister since 2006, I think.
Besides, he's the most atrocious public speaker. Have you ever heard him? Embarrassing to listen to.
There's a possibility that he'll be the next Chancellor but one, or maybe two, but not until he's shown his mettle in another cabinet job first. I could see him going to Chief Secretary, then Work and Pensions and then maybe, Chancellor.