As leader he must distinguish himself and his party's plans for deficit reduction from that of the coalition Government.
Ed Balls - the frontrunner for shadow chancellor - has already said the speed and severity of the cuts are 'unfair and unnecessary'. But Balls, arguably tainted by his closeness to Gordon Brown, hardly echoes change, even if his anti-cuts rhetoric has played well with disillusioned Liberal Democrats.
So, appoint him to his coveted role or break with the past?
Miliband then has the challenge of opposing some of the cuts at the same time as rebuilding his party's economic credibility - a nuanced position requiring deft comms handling.
Some suggest he will retreat into an 'old Labour' comfort zone: but he will surely defy the sceptics and quickly move to demonstrate his independence from the unions and willingness to appeal to the centre ground.
The big quandary he will face is the prospect of public sector strikes: testing his willingness to challenge the very supporters that secured him the leadership.
But the key areas are those coalition policies opposed by the Labour grass-roots and only reluctantly supported by the Lib Dems: renewal of Trident, tuition fees and the VAT increase spring to mind. Strong opposition to these, and others, will help to hold on to those recalcitrant Lib Dem defectors.
That's for starters: he will have to go much further if he is to win back the million voters who deserted Labour in May. The party may have decided that Ed Miliband can win back Labour's disaffected core vote, but it's the centre-ground that will deliver the southern marginals.