Choosing how best to cast your vote under AV is actually considerably more complex than many imagine. I agree with the proponents of AV in that it is relatively easy for voters to rank candidates in preferential order. This is not the issue. The problem with AV is that it creates circumstances in which people should strategically cast dishonest votes.
Under FPTP the only way a Conservative supporter can ever help the Conservative candidate win in a constituency is by casting a vote for the Conservative candidate. It is true that many voters decide to give their support to the candidate most likely to oust the incumbent as opposed to their preferred candidate. Nonetheless in these circumstances they still make their vote understanding that they will increase the vote total of the candidate they voted for by one.
Under AV however, there is the possibility that by casting your first preference for the Conservative candidate, you will cause a Liberal Democrat victory; whilst by casting your first preference for the Labour candidate, you will cause a Conservative victory. That’s right: vote Conservative, get a Lib Dem MP, vote Labour, get a Conservative MP. Don’t believe me? It seems too ridiculous to be true doesn’t it?