IMO it'll be the 'George Osborne' factor that strains this coalition more than anything else. To me, the dream ticket was to have Clarke back as chancellor with Cable as his number 2 and a sideways move for Osborne to Business Secretary.
This is the one area I believe cameron hasn't acted "in the interests of the country" but in the interests of loyalty to his friend (that said, loyalty isn't a bad facet for the leader of our country to have)