Often what is flawed about affirmative consent proposals is that they are vague on how they are all that different from established consent law, introduce a lot of subjective interpretation to an issue where present law is fairly clear cut (though difficult to determine for obvious reasons), and subverted just as easily as current consent laws: lying. Due to all of this they just sort of strike me as the sort of laws you implement in a "We must do better!" moral panic and don't really think them through very clearly.
^This^
I obviously agree that there should be an expression of consent to initiate some sort of sexual interaction but am uncomfortable about explicit verbal statements of consent being the only way. It ignores a lot of the way that we as humans communicate. Body language, nodding the head, and shouldn't you being the one initiating the act be implicit interest in the activity? And there are people with disabilities that have difficulty with speech, or the deaf. There are a lot of things that complicate that being the only standard.
^And this^
I basicly agree with everything Marokai has written on this topic so far.