On some issues, yeah you get that kind of blunt ideological approach, but on an issue like - the effectiveness of the two men, how strongly they would be polling - I would have suspected there would have been a stronger current of recieved wisdom at David's effectiveness, perhaps born out of cynicism at the foibles of the electorate.
I don't doubt that The Next Leader Of The Labour Party was in for a rough ride no matter what, even the left-leaning press had to slate them to fit the national mood after Brown. Let's look at them and the likely press response:
Ed Miliband OTL. Thanks to circumstances (a factor we kind of have to ignore with the ATL options below), seen as illegitimate, 'in the pocket of the unions' and backstabbing his brother somehow. These three evaporated after about 12-18 months but the general sense of unease at his speaking voice and apparent lack of manliness remained and remains the primary route of attack.
David Miliband Iraq, torture, neo-Blairite (however accurate or inaccurate this may be), Iraq, torture and an incomprehensible wonk. This last criticism has been aimed at Ed but I think Thande is right that because David is better looking (which ain't saying much, I grant you) and a bit more presentable, it would in fact be
more offensive when he ends up talking like a Sam during Season 2 of the West Wing. Also expect plenty of 'he didn't have the bottle to challenge Brown' accusations, and the Libyan torture story wouldn't go away as quickly as it did IOTL.
Diane Abbot Tricky, because the two obvious -isms that the press wouldn't want to be open to being accused of. Luckily for them, Ms Abbot has a gaffe-loaded history and if she had somehow become leader of the Labour Party, speaking bluntly she would very quickly be exposed as out of her depth and apart from scoring some cheap points about not being a white guy in a suit (admittedly she definitely wouldn't have the 'they're all the same' problem that the other candidates have against Cameron), she would be regularly schooled at PMQs and probably get a lot of things embarrassingly wrong in the heat of the moment - when I saw her live she said she thought 'Britain' would win the World Cup, and in the Newsnight debate she attacked David for being Foreign Secretary during the Iraq War (something that, amusingly, Ed corrected her on). She'd write the headlines that would hang her, frankly - think Nicola Murray.
Ed Balls 'We just got rid of Brown.' The bruiser himself would not have an easy time of it. After only just holding on in Morley and Outwood and denying the press their Portillo Moment, stories about briefing, McBride and 'how he's basically Brown' would be the main line of attack. There's a reason the wags/pub bores/wankers like Old Holborn or Guido ran an ironic 'Ed Balls for Labour Leader' campaign. When you look at the criticisms he's had as Shadow Chancellor, it would be very difficult to see them not being leveled at the party as a whole and Cameron scoring win after win at PMQs by accusing him of having no credibility, no plan etc etc. This might dry up after a while, but Balls' greatest threats have always been within the party, not from outside it, and the press would relish a return to the days of 'who's briefing against who this week?'. He'd probably have the toughest time of it from the press.
Andy Burnham Despite doing well to secure the Tindale vote, everyone's favourite lower middle class Cantabrigian (but he's still got a northern accent, so he's working class, guys) would probably be smacked in the face with a royal dose of 'who the hell are you?' and 'Labour elects no-mark with the eyes of a cartoon female dog'. My own reservations about how he presents himself aside, he wouldn't be accused of being a posho and would compare well against Cameron. But the line of attack would be more on his inexperience (something which would have been the main one against Ed had he not had a brother) and somewhat flaky style of operations. Probably would have been able to pull through, and I don't know enough about his standing in the party to know if he'd maintain support (but if they didn't ditch Ed they'd've kept him) and he might have ended up being pretty accepted and gained a lot of brownie points in the wider picture by making opposition to NHS privatisation the centrepiece of the Labour platform, which could have become his 'Hackgate moment'. He might have done alright. But he was running so he could run and win in 2015 in the (then likely) event of a Labour defeat and departure of *Miliband, and his credentials at the time showed it. Would score well on being 'in-touch' though - when he was asked the same question as Ms Abbot above, he was the only candidate who actually said 'I'll of course be supporting England, but I think Spain have a real shot this year.'
So that's my take on how it would have gone down for the contenders we had.