I found that once I had DS I was able to see many, many similarities between myself and other mothers. It was quite a humbling and inspiring side effect of having a baby actually!
For example, sitting on a plane next to a woman from Abu Dhabi who had a son the same age as mine - with our babies there we were able to communicate, support & make each other laugh without much in the way of a shared language, and coming from different cultures / religions / races (some of the most obvious "differences" between groups of people I suppose)
I have lots of male friends, I suppose the similarities between me and them is often things like our sense of humour, we've experienced similar things in our lives, interested in same books, strong sense of principle, caring about other people etc. I wouldn't say I 'pick' my male friends using a different set of criteria from those I use when 'picking' my female friends.
Definitely agree that men and women display certain traits on a spectrum, and the 'weighting' of that spectrum is very much influenced / imposed by society. TrillianAstra's example of height difference is excellent and I will steal it next time I get drunk and talk to my FiL about feminism!
Another recommendation for the Cordelia Fine book, by the way!