The most important difference of copper is an easy accessibility
of four oxidation states from 0 to +3, while palladium
has at its disposal only two stable oxidation states—0 and
+2. There are indeed +1, +3 and +4 oxidation states for palladium
[10], but these are either extremely rare or play no
unambiguously identifiable role in cross-coupling reactions.
Most likely, the cross-coupling catalytic cycle with copper is
serviced by +1/+3 oxidation states.