This was a feature of the 'Politics of Respect' TL I did a while ago, although it wasn't the premise.
Basically, we would see many of the measures brought the Tories carried out in the 2015-16 period brought forward as part of an accelerated program of austerity, and you'd see the kind of deterioration in public services that we have seen over the last few years a little early. We know that Clegg vetoed things like Osborne's changes to child tax credits, the trade union act, cutting the top rate of income tax to 40p, scrapping the Human Rights Act, a free vote on fox hunting, and I think also an unlimited top rate of tuition fees.
I don't buy that all of those things would be implemented if the Tories had a majority, as some of these things might have been floated by the Tories to gain leverage on the Lib Dems, but I think its safe to say at least a few of them would get passed, as they did later IOTL. That would undoubtedly leave the Tories more vulnerable than IOTL. Gay marriage would most likely still happen though-it wasn't in the coalition agreement, and it was something Cameron had been personally in favour of beforehand. The votes to get it passed would still be there.
As for Labour, I'd agree that David Miliband stands a higher chance of winning the leadership in this scenario if devouring the Lib Dem vote does not look like a viable path to victory at the next GE. Without that, Ed might not even be convinced to put his name forward. The one thing I would note is that the extra MPs Labour have lost in this scenario would come disproportionately from David's camp-so that might cost him a couple of points in the electoral college. Another interesting development would be that Ed Balls loses his seats five years early-which probably might mean Cooper runs in his place, thus preventing Abbott from being on the ballot as the only woman candidate. Certainly, Cooper would have a stronger claim to the role of Shadow Chancellor.
The Lib Dems would have a brighter future ahead of them-although quite how bright would depend on whether there were any debates in this scenario. Without them, Clegg would be a somewhat popular but also fairly unknown party leader-who would have questions hanging over his leadership after a loss of seats, but with them, he would be a highly popular figure who would have a similar kind of profile to the PM and Leader of the Opposition. That would prevent the LDs collapsing-and they'll probably win the Oldham East and Saddleworth a few months into the parliament. They'd also be far less likely to see their vote collapse in the 2011 Scottish Parliament Election, which would mean no pro-indy majority, and no referendum-which would mean no huge SNP surge for the Tories to throw at Labour-although maybe we would see a majority for a referendum in 2016 instead.
Though they might not ultimately poll so well, UKIP would still rise, and still probably take a lot of LD votes as well as Tory and Labour ones. Cameron would still pledge a referendum, still probably after 2015 in order to bring eurosceptic voters into the Conservative camp. My guess for the outcome of the next GE would be another hung parliament, with Lib Dems and Labour making enough gains to form a government. David Miliband would become PM, probably with Clegg as Deputy, in return for a referendum on PR, and various other things-any changes to tuition fees would probably be towards reducing payments or some sort of graduate tax, rather than scrapping them altogether, so there is still the potential for some sort of Lib Dem betrayal narrative to develop. Austerity would still be taking place, albeit in a less extreme form.
Cameron would resign, and be replaced by someone who pledged a referendum on leaving the EU as part of there platform-most likely Boris.