I totally disagree that "assertive but kind" is a rare trait amongst the "technically proficient". Anecdotally, I know plenty who exhibit all of the above in spades.

The key is culture. Technically proficient people working within a community where "assertive but kind" is the norm tend to follow that norm.

I'll mention Doug Cutting as the ideal who melds technical proficiency, assertiveness, and kindness. I've often thought of Doug as a better role model for Open Source leadership than Linus Torvalds -- but it looks like after today I'll have an opportunity to reassess!