Janan Ganesh in the FT (a former university classmate of this writer) makes an important point (part paywall) when he says that we do not yet know what the electorate meant with the General Election result. I would go further. I think it is impossible to know what any electorate really means as long as First-Past-The-Post is the voting system.
We in Labour are as guilty as the rest when we rely on tactical calls in specific constituencies. 'Can't win here' leaflets are a regular campaign feature, telling people to not waste their vote on those they actually want to see returned to Parliament.
These calls work. A Green-supporting friend of mine voted for them for the first time this election. He was bored of forever compromising with his vote, only to see his mis-cast cross claimed as a 'mandate from the people'.
My issue is this: no clear or obvious mandate can exist when so many are already casting second or third preference votes. Politicians cannot both ask people to negate their first choice in favour of influencing the outcome, and then claim this vote as proof of their true belief. There may well be a majority in favour of conservatism - particularly if Conservative and UKIP vote shares are added - but this electoral system will never tell us. An electoral system that encourages people not to reveal their true preference will always lack legitimacy.
Furthermore, this barrier to entry to new parties prevents the one thing - competition - that would actually provide a feedback loop to the two national parties. They will never truly change, as they are institutionally protected in their electoral duopoly.
The constituency-MP link is important, and this can be retained with, for example, AMS, STV, or AV+. However, I would make a further point. I am a Labour Party member (and unsuccessful council candidate) in Windsor. My MP is a Conservative, Adam Afriyie. I do not feel represented by him. Indeed, in three sentences when we met for the first time last Thursday (outside the Eton Wick polling station) he was deliberately antagonistic towards me as the Labour Party candidate. I would not trust him with a sensitive issue, and this is not unreasonable for those involved in street-level politics.
It would be better to have a choice of representative, perhaps from a multi-member constituency, so that I may choose a politician that I feel comfortable with representing me. The vast majority of MPs, lest we forget, are elected by a minority of their constituents. They may say they will represent all, but that does not mean that all would wish to be so represented.