Heretofore, when writing about the Nice Guy phenomenon, I have stuck to one line: a Nice Guy is someone who thinks he has the answer to What Women Want, and doesn't understand why women ditch or ignore him for men he calls "assholes," meaning men who realize that women are individuals and want three billion different things.
What it comes down to is that women are not machines. There's no input that guarantees the result of sex. Women have the same sexual autonomy as men do, and nothing -- not putting a woman on a pedestal, not getting her favorite wine/flowers/porno, not standing outside her window with a boom box -- imposes any sort of obligation on any woman to sleep with you, or date you, or anything else. A Nice Guy isn't simply someone who doesn't really know how to treat women1, but anyone who thinks it is necessary or desirable to obtain sex (a loaded phrase right there) by being manipulaive, often deceitful.
Thus to a certain extent my version, too, strips women of their personhood. Finding out what a particular woman likes is no more an automatic in than going down some generic checklist of Things Women Like regardless of its applicability to a particular person.
So, the new version: A Nice Guy is someone who thinks he can obligate women to sleep with him by behaving decently, however he conceptualizes that, and behaves decently for that purpose rather that because he's genuinely decent. This Nice Guy may still have an expansive definition of asshole, but he also doesn't really know how these men actually behave. Most are still probably as decent as the Nice Guy,
and more interesting.