So, that's it.
To say this is a stunning conservative victory is putting it mildly.
The simple truth is that no-one saw the Conservatives increasing their number of MP's, let alone winning an overall majority. And why would they? The last election... which featured a remarkably unpopular Labour government and came in the midst of a financial crisis... saw the Conservatives fail to win one and the idea that they would now when they're in government and have gone through with a pretty unpopular austerity regime was seemingly ludicrous, especially with the suggestion that UKIP would split the right wing vote. As a general rule governments lose seats at elections... the last one where the number increased was back in the 1980's... and with a number of very marginal Tory held seats it was fully expected that the Conservatives would at best stand still. Instead they've triumphed. Looked at in isolation this is far from a crushing result; 331 seats (the predicted result) is a fairly slim majority. But in context it's massive. The Liberal Democrats have been virtually wiped out, losing almost all of their notable MP's. Labour have been badly bruised, losing some big hitters of their own (notably Ed Balls) and having their power base in Scotland destroyed. UKIP got the votes (12% in total) but not the seats and the evidence suggests they took more votes from the Lib Dems and Labour then they did the conservatives.
So, why did it happen?
Here are my brief thoughts.
The SNP clearly did remarkably well in Scotland, winning all but three seats. This had the obvious impact of getting rid of about 40 Labour MP's but that didn't make a massive difference in and of itself; an extra 40 MP's wouldn't have won Labour a majority. I think the effect was somewhat more subtle then that. As the campaign went on and it became more and more clear that the SNP would do well Sturgeon and Sammond became more and more bellicose about what that would mean and the power it would give them. If the results had gone as previously expected then a Labour government could only rule because the SNP decided to support it and the SNP were open about the fact that they'd demand huge benefits for Scotland as the price of that support. The Conservative campaign picked up on that and I suspect more than one undecided voter made their final decision to vote Conservative rather than Labour in marginal seats as much to keep the SNP out as to keep the Conservatives in.
2) The Economy
It hasn't actually been the biggest focus of the campaign but with a first term dominated by the financial crisis and the austerity policies that followed it was never far from the mind. The Conservatives were aided by the fact that over the past year or so the economic news has generally been positive (which seemingly indicates their policies are working) and that other countries in Europe who took a slightly more left-wing approach are struggling (which makes the Labour criticism look inaccurate). Perhaps more importantly though the Labour party still struggled to get to grips with its own time in office and role overseeing the start of the crisis. A large number of its most high profile members were also high profile members of the Brown-led Government and they were never able to distance themselves from it and the sense that they still weren't trustworthy with the economy. An attempt to defend their own record was an attempt to defend Brown's record and the idea that the Brown government massively overspent has been fixed in the public's eye.
3) Milliband himself
You may think it's unfair to mention, but Milliband struggles to appear Prime Ministerial. He comes across as the person who thinks up policies and writes speeches in back rooms, not the one who delivers them on a podium. Frankly, he's just awkward, fairly uncharismatic and lacking a real presence. You may think that doesn't matter... but it actually may well do. Could he stare down Salmond and Sturgeon? Could he face off with other European leaders? And how would he fair if stuck in a negotiating room with Putin? Furthermore the manner of his ascent to the leadership of the Labour Party and the perception that he stabbed his brother in the back to get there stuck. We all know that party politics if full of shabby deals and treachery... we just don't like to see it.
4) Lib Dems and UKIP
Most expected the Lib Dem vote to collapse, even if not dramatically as it actually did. Labour hoped that most of those voters would come to them and that in turn UKIP would split the ring wing vote, hurting the Conservatives. Neither really happened. The Lib Dem vote didn't go wholescale to Labour and UKIP seemed to pick up most of its support from it. In seats where the Lib Dems and Conservatives were primarily competing the Torys won and Labour were reduced to third place, in places where the Tory's and UKIP were neck and neck the Tory's won and in places where Labour and the Conservatives were close voters turning to UKIP wasn't enough to give Labour the edge. Some of UKIP's best results came not in the Conservative dominated south but the Labour held north and repeated a trend we saw previously with the BNP; while both it and UKIP were seen as right wing parties their generally negative take on immigration finds much of its traction with working class voters who were previous "old" Labour supporters living in industrial heartlands.
Where does that leave us?
Cameron is the clear victor today and the buzz from managing this success will last a while. But he needs to be careful. A slim majority gives backbenchers more power and the Conservative backbenchers have tended to be a troublesome and rebellious lot. Today they may be utterly loyal and supportive as Cameron delivered the majority they craved. But two years from now? Especially once the discussion on the EU comes to a head? Expect rebels.
Labour need a new leader and face a tough choice of where to go. Milliband tried to distance himself and the party from Blair and Brown but never really managed it getting all of the downsides with none of the positives. Do Labour react by going more to the left? Or do they head back for the centre and the ground that Blair once so skillfully occupied? The Lib Dems have been destroyed but the question is whether this is a one-off reaction to the coalition (and so the voters may return next time) or a longer lasting blow.
Even the SNP have concerns. Their campaign has been a triumph but it was a campaign based around "standing up for Scotland" and the power that they would wield in Westminster. With a Conservative majority they wield no power at all. Cameron appears willing to grant their wish of more devolved powers going to Scotland... but with his majority he can dictate the terms and method of those powers. Looked at utterly cynically Cameron may wish to give them everything they want because a strong SNP hurts Labour (for the reasons mentioned above) but they would be far from the first party to find that power and responsibility is not the golden goose they hoped it was... look at the Lib Dems this time.