The fact that he didn't have a warm coat is absolutely irrelevant. We had high-quality intelligence against him but failed to act upon it in any way. We need to worry less about whether someone looks shifty at an airport and worry more about keeping tabs on people who we have strong intelligence on.
It is also worth stating that his attack failed and he was unable to damage anything other than his own groin.
The recent cargo bomb was missed despite multiple tips about that specific item as well!
http://en.wikipedia.org/wiki/Umar_Farouk_Abdulmutallab#The_a...
Terrorist attacks increase the funding of the intel and security institutions, so they have a vested interest in them occurring (and a state willing to kill tens of thousands of civilians in Iraq to advance its objectives likely has little problem sacrificing the odd planeload of its own citizens to maintain public support for its ambitions).
Right, you're carrying something. It seems like the Christmas Day bomber wasn't carrying anything (aside from his bomb).
So, the red flags are noticed at least sometimes.