Balls is dangerous because he is such a natural at Opposition, and he isn't afraid to knife anyone, using the dirtiest of dirty tactics - as we saw with his continuing friendship with Damian McBride and Charlie Whelan, his part in the downfall of Tony Blair, and of course his part in ousting Alan Johnson. This same malicious streak is also a massive liability for Labour though, because no-one, even within that party, actually likes him.
He also has the distinction of being one of the main authors of Gordon Brown's disastrous economics - and he won't be allowed to forget it.
Labour have been deficit deniers for ages now - but at least some of the more Blairite ones gave the pretence of caring. Balls wants more borrowing, more taxes, more spending. He is about as pure a deficit denier as they come.
So there are both dangers and opportunities for the coalition. We live in interesting times.