I've finally figured it out. The solution is rather simple, which explains why I didn't think of it before (turns out that simple answers are hard to find)
First of all, you may know the problem: a girl1 complains that she wants to date a nice guy, that his boyfriend is an asshole or she just compliments a guy saying that he's nice and how much she'd like to date someone like him. The "Nice Guy" problem. If you encounter this kind of girl, either stay clear and wait for her to come to a shocking realization of maturity2 or flee as fast as you can. She's (likely) not the girl for you.
When a girl says "I'd love to date someone nice / as nice as you" what they actually mean is "I'd really prefer that whoever I'm dating also happens to be nice as you". They want whoever they like to be nice, but it's not a defining trait for someone to be dateable before her eyes. In other words, niceness is sufficient but not necessary3.
"So what?" you may ask "What's so bad about a girl wanting his date to be a nice guy?". Nothing, per se, but the way it's presented/interpreted gives way to unnecessary problems. Let me explain:
When dicussing something crucial (like feelings) in a trivial way, there will always be some kind of degradation of the message or its meaning, something will be missing either on purpose or by accident. In this particular case, a crucial thing (the desired traits in a significant other) is talked of in a light, trivial way4. A girl says that she wants a nice guy without really knowing what that means for her, just reacting to niceness, similar to the way the smell of pizza can make someone crave it even if one doesn't have any hunger at all.
Men also contribute to this problem. The Nice Guy Problem arises when a male identifies that a girl wants "a Nice Guy" and goes out of his way to become that one Nice Guy. Being nice to others is good, but forcing yourself to niceness to get someone else's attention is not. More importantly, forcing someone else (actively or passively) to like you is not healthy. It tends to create the idea that all women are soulless bitches who use nice guys and that all nice guys are martyrs worth dating5.
Then it makes sense to say that a good portion of the girls who claim to want a nice guy are not stating a real desire or need in their relationships; they're reacting to niceness and will most likely not act on that premise. These girls just want a hot boyfriend who is also nice (notice the order of those adjectives).
I'm not blaming these girls. Getting to know oneself is a hard and very long path. Not everyone wants to follow it and not everyone that follows it will endure it for long. These girls are just beginning to notice that niceness is a desirable trait and it's up to every one of them to decide how much importance to give to it.
I'm not blaming (nice) guys. They too are reacting to what the girls say and acting on an impulse to fulfill what they say they want. Nice guys also have to remember that the first person you have to like is yourself and that doing good for others only to impress them is the polar opposite of being genuinely Nice.
There are girls who say they want to date a nice guy and then there are girls who actually date (and often marry) nice guys. The girls who really want a nice guy will actively look for that quality (however they may define it) in the guys they date but often will not say it out loud. These girls are often mature enough to realize that hotness goes away and niceness can grow indefinitely. These girls are often clever enough to date guys who are genuinely Nice, the ones that help others out of a desire of making the world a better place and not to get someone's attention.
She doesn't really want a nice guy, so don't worry about it.
1 I'm writing this from a heterosexual male perspective because that's what I've experienced and as such is easier for me to think in these terms, but I suspect these problems can happen to anyone regardless of their gender and sexual orientation.
2 Which may take years if it ever happens at all. Some girls will never come to this realization.
3Sometimes this also applies to other cuasi-desirable characteristics, including but not limited to "a guy that plays guitar", "a guy with good grammar", "a guy who's into girly things" and so on.
4 Not necessarily out of willful trivialization of one's feelings. Realizing one's likes, desires and wants is a quest that can last a lifetime.
5 Note that this isn't necessarily a lie. The generalization is the problem here: Some girls are soulless bitches but not all. Some "nice guys" are worth dating, but not all...