1) Firstly, the big one. This could well be studied in the history books as an example of parliament radically extending its control over the executive. The days of commentary about parliament's weakness during Tony Blair's time in No 10 seem like a distant dream. To give you some impression of the extent of the change we're dealing with, the best example I can think of is from the years just before the English Civil War, when parliament demanded war powers. (I'm not a historian so feel free to correct me if I'm wrong on this). Since then it has been accepted that the executive did not need to heed it on war and peace. Even at the start of this week, it was said Cameron could really move forward in Syria regardless of the parliamentary vote. It will now be very hard – impossible even – for a government to go ahead with a military engagement without first consulting parliament. Under the likely currents of Britain's informal constitution, parliament has just got itself a veto.
2) Britain is not taking part in the Syrian mission. America may have France – maybe even Turkey and Italy and Germany – by its side. But Britain will not be going. It would be political suicide for Cameron to try to hold a second vote. He lost this one, and this wasn't even directly on intervention. And going ahead without a vote would be politically impossible. The UK, for once, will not be involved in a US-led military adventure. Our whole relationship with America could be about to change.
3) Cameron's authority has taken another hammer blow. After a good summer of Tory MPs feeling cheerful about the coming election and Labour in the doldrums about its leader, all the old headlines about Cameron's weakness and inability to control his party have come back to haunt him. He is back where he was a year ago. And worst of all, this is self-made. He walked right into it.
4) Iraq has poisoned the British willingness to participate in military intervention. It was the shadow of Iraq which guided today's vote. It subdued the public appetite for foreign wars in a way that was unthinkable after the Falklands. And without that public support – or even division – MPs felt more confident standing up to the government. Ten years after it took place, it's becoming clear how much of a game-changer that conflict really was.