I hate radical feminism as much as any sane person (radical feminism being a specific category of feminist ideology; the militant branch who DO think all men are evil and are serious when they say they want genetic engineering to make men obsolete so that all men can be euthanised without the species dying)
Just to be clear, this isn't the radical feminist theory which is today core to most feminist schools of thought.
Radical feminism is, at it's core, based on the concept of the patriarchy. The idea that men have structured society men having authority over women and that inherent power structures greatly favour men. The term radical comes from the civil rights movement, whose methods and structures were borrowed by certain groups of 60's feminists. Most of the serious social pushes in feminism came out of the radical movement, including things like the Equal Rights Amendment and abortion rights movement, broadening the focus of earlier groups that had concentrated on economic matters.
I doubt you will meet any feminist who doesn't hold views representative of at least some factions of the radical movement, especially in the more popular modern mainstream feminist movement. The movement has evolved (especially on issues of sexuality and gender identity), but many of the core ideas are now fundamental building blocks of the wider social justice movement.
The term does not apply to fringe cranks. While there were radical elements who advocated replacing the patriarchy with a matriarchy that is not representative of the radical movement as a whole and the term is way too useful as a political label to marginalise it that way.