It strikes me you’d rather see it from the horse’s mouth, so here’s a choice extract from a message Rich posted:
Scheme #t is almost completely
meaningless, as Scheme conditionals
test for #f/non-#f, not #f/#t. I don’t
think the value #f has much utility
whatsoever, and basing conditionals on
it means writing a lot of (if (not
(null? x))… where (if x… will do
in Clojure/CL, and a substantial
reduction in expressive power when
dealing with sequences, filters etc.
The links in that message are also worthwhile, though the second one may be a bit poetic.