Labour lost because of three main reasons: Leadership errors and appearance, the SNP surge and a very successful negative Tory campaign. Labour must work out which reasons where the most significant so that it doesn't end up breaking things which work.
Many people blame Labour's defeat on Miliband, primarily his appearance. While he did turn off many voters and, sadly, due to the importance of image in Western society his appearance did damage Labour's fortunes, that wasn't the only problem.
Miliband and his team failed from the get-go. They didn't shout loud enough that the recession was not caused by the deficit, which is a nonsense suggestion economically, but by financial deregulation which the Tories supported and which Labour repented of. This blog has mentioned a number of times this argument, but Labour only started to counter the argument towards the end of the campaign and by then it was too late. Even then, Ed didn't communicate the argument to the Question Time audience that asked it clearly enough and it didn't really hammer home.
Of course, Liam Byrne's 'I'm afraid there is no money' letter didn't help this at all. Although it was meant to be something of a joke in light of Labour already making savings in 2010, the damage was done and Byrne has apologised for this. However, Labour should've pointed out earlier its savings, its record and how meaningless the letter was before allowing Cameron to destroy its record.
There was a deeper problem however. In a sense, Labour couldn't defend its record because it broke away from New Labour and wanted to put some distance with its past. This turned out to be a terrible mistake. The public voted Blair's New Labour into power three times and it was only after the recession and the failure of Brown to defend his performance in saving the UK (and indeed the world) from the most of it that led to New Labour's defeat then. Cameron's centrist appearance didn't help either, of course.
All the 2010 Labour leadership candidates wanted that break with New Labour so that wasn't just Ed's fault, it was a fault in the party's thinking. We listened to the voices that said that Labour needed to appear to veer to the left to differentiate from the Tories and we took action - with disastrous results.
It's about appearance with centrism and leftism for Labour because Labour policy is developed by YourBritain, the National Policy Forum and the National Executive Committee, not really the leadership. However, the leadership sets priorities (on the whole) and presents the policy package in the way they want. They decided (as others have said elsewhere) to go on a less pro-business platform and focused on attacking vested interests which, while important, gave them a negative, anti-status quo image. They should've stressed more their policies on guaranteed jobs, apprenticeships and making work pay earlier in the campaign. However, this was undermined by their loss of the argument on New Labour's spending and their anti-New Labour stance.
Of course, the SNP and Lynton Crosby's campaign didn't help (Miliband was wrong there too, Crosby did very well), but there's obviously a lot Labour can do. A more popular leader, renewed focus on helping middle-class people and defence of New Labour would help - as would maintaining Ed's return back to fighting inequality.
Therefore, over the next five years Labour should fight on a platform of helping the middle class succeed and assisting those who suffered under Tory cuts. Twin aims, two focuses. The balance they take and their success (or lack thereof) of defending Blair's record will be the decider of the next election.