I think one way in which the "extremism" of radical feminism could be characterised is that it regards the existence of two social classes men and women, respectively the dominant class and the subordinated class as the single fundamental explanatory fact about society, the fact in terms of which all social institutions are to be analysed. Other feminisms would allow the fundamental explanatory power of other forces. For example, socialist feminism holds that at least some of the time (but not necessarily all of the time) gender relations supervene on economic relations (of ownership and non-ownership), and don't themselves determine economic relations
A problem with identifying the rad fem view as "extreme" seems to be that it creates an impression that radical feminism is just feminism taken to its fullest extent, as if there were a pre-ordained feminist journey, with a single direction and with individual feminists just choosing to stop off and make camp at their preferred point along a single road. That's how you get the unhelpful idea that the radical feminists are extremists and/or that the rest are "feminist-lite" -- uncomplimentary to both sides.
That picture relies on a false idea that there are just two feminist orientations, "liberal feminism" and "radical feminism," and hence a single spectrum, when in fact of course there is a whole family of feminist orientations, to which the idea of "more extreme" or "less extreme" doesn't always apply.
As well as rad feminism and liberal feminism the family of feminisms also includes at least the following (I'm sure other posters could suggest more):
--socialist feminism (no less radical than rad fem, but with different explanatory categories that are incompatible with rad fem)
--difference feminism (with incredibly interesting ethical theories)
--critical race feminism/third wave feminism
--postmodern feminism
Only some of those are mutually incompatible, and it is possible to identify with more than one of them. I really really hope that discussions on this board can now be fluid enough to match the fluidity of feminism itself.