Let me take two right wing policies as indicative. In real life it may be different ones. Let us say the removal of collective bargaining rights from public workers, and the privatisation of the NHS. Let me further assume that these are policies which do not have general public support (as I think they do not). With AV such policies could become law through a simple tactic.
The Lib Dems and the Cons publicly agree on the removal of collective bargaining, but publicly and loudly disagree on the destruction of the NHS. The Lib Dems then run a campaign at the election which says to the mass voter 'I know you don't agree with all our policies, but give us your second preference votes if you want to save the NHS.'
Under AV they pick up a few marginal seats in this way. They then form a coalition with the Tories and close down both collective bargaining and the NHS, shrugging and saying 'this is coalition politics, we can't achieve all the items in our manifesto.'
If this tactic has occurred to me, you may be sure it has occurred to people who - unlike me - want these outcomes.
Obviously if the AV referendum does succeed people like me must make a lot of noise to fight this tactic, and persuade people not to innocently enable policies that they will hate, but it is an additional big risk, and probably the main argument against AV.