Since there seems to be several ideas of the "friend-zone" here's mine: Guy/girl is friends with someone. Said person begins to see them in a romantic way and believing there is a chance, most often due to comments from the other person such as "You're a great person!" or "Anyone would be lucky to be with you.". Said person attempts to pursue this, not actually asking them out from nowhere but by presenting themself as a romantic option, and is shot down, despite being what (s)he claims to want. The other way is someone who honestly rejects the friend to avoid any complications to the friendship, but from what I've seen that being the real reason is kind of rare.
In my view of it, it only becomes the "friend zone" (other than the short literal one I said above) after presenting yourself as a potential partner instead of a friend, and after some kind of indication from the other person that they think highly of you and may be interested. You can't blame a person for shooting you down that shows no interest whatsoever. I also think we need avoid making this out as such a bad thing, because if you were friends first and became romantically interested afterwards, then all the qualities you like are still there and you still have your friend. Endless moping around about it is what people don't like. That isn't saying it doesn't suck to have one-sided feelings, but don't make it seem like the world hates you for it. Pick yourself up, enjoy your friend, and get back out there.
EDIT: To clarify, there isn't really any blame to be had in these situations. That's just how it's going to be, and you can't blame someone for not being attracted to you the same way you are to them. Unclear signals and poor communication tend to cause the problems, so just be direct.