> Are there any tenets of secular humanism that extropians or
> transhumanists outright reject?
Transhumanists obviously reject the meek acceptance of man's
mortality and general mental and physical limits that is typical
for humanists. I don't know whether or not it's an actual tenet, but
afaik humanists generally belief that things like immortality are
somehow "immoral". Unlike transhumanism, secular humanism gives
nothing in return for the promise of heaven it has stripped away.
Also, I often find people who call themselves "humanists" to be
a bit too far on the bleeding heart liberal side (soft treatment
of criminals and the like -- yuck!).