Sucking is absolutely essential to good head, in my view, no matter whether it's being performed on a penis or a clitoris.Not if you're doing it right.
Other than 'mouth on genitals' there are no similarities that I can think of.
*thinks about it some more*
Nope. No significant similarities. Penises get licked, but, yeah, that's it.
(Obviously we all know what we personally like, so if you don't like being sucked I don't mean to "correct" you, but if I were going down on someone I'd always be planning on sucking)