So what happens when you miss the QTE? i missed one in the very beginning (where you have to kiss that chick) and i thought that it was cool that the scene continued but with a different action (she kissed Ezio's neck instead of lips). but what happens if you miss some of the more important ones? like when you have to stab those guys that are holding your arms by pressing x? did anyone do this out of curiosity?

I'm not sure about that specific scene, but if you miss it something else will happen. I missed the one where you walk into your house and that girl tries to hit you in the head and all that happened was she hit me in the head and I fell over.

No.

It's just gestures you do that correspond to pressing the button. I missed one once where da Vinci wanted to give me a hug and I had to press a button to initiate said hug, I didn't press it and a quick awkward moment passed, which was kinda funny.

to be honest, i thought the QTE's were rather weak in AC2, I felt that they were rather awkwardly placed even if they did add some nice interaction to the game.
 
maybe its just that im used to sitting back during cutscenes... :/