I am far from a British politics expert, but here's my take:
If Labour gets in with a coalition, cuts and measures to bring down the massive debt will reflect badly on Labour and the coalition they'd need to assemble. On top of the right wing press playing this as Brown (or replacement PM) camping out in 10 Downing and stealing the election.
If the Lib Dems support a Tory minority government with no coalition, this may tamper some of the most horrible cuts, but many supporters (including myself) would question future support of the party. The Tory press would blame the "loony Left" for unpopular measures dealing with the deficit and the coming out of recession, and if Cameron had been given an outright majority of seats, they could have implemented better policy. Of course, even if Cameron had won an outright majority, things that they did that harmed the country would not be the Tories' fault even if they could admit that harm was going on...
Labour supporting electoral reform now...too little too late.