Well, I tend to think of "bottom" as a term associated with childhood language, hence the strong appeal of "bare bottom" in a spanking situation, since the spankee is being treated in a manner that most of society would consider to be childish. I love the idea of a physically powerful adult (or near-adult) male being sentenced to a "bare-bottom spanking" by a smaller, weaker female who nonetheless has the uncontested capacity to soundly spank him--conferred by her position of authority (formal or informal), greater emotional maturity and/or dominant personality.
Other terms for the buttocks which I consider rather child-oriented are "fanny" (in the U.S.A. anyway), "hiney," "tushie," "bum" (or "bummy"), "cheekies" and "nates."
Somewhat clinical terms, other than "buttocks" itself, would be "posterior" and "hindquarters," in my estimation.
Widely used but fully accepted general terms, other than "bottom," would include "behind," "rear end" (or just "rear"), "hind end," "derriere," "caboose," "buns" and "seat," I'd venture.
"Butt" and "ass" strike me as a bit more crude, I tend to use them in seriously severe disciplinary situations.
I'm also fond of variations of "cheeks," as in "asscheeks," "bumcheeks," "buttcheeks" and "nether cheeks" (or even "nether moons").
Occasionally I employ rather improvised, purportedly individualized terms such as "south side" or "southern hemispheres" in my spanking-oriented stories.
Certain synonyms for spanking work better with particular terms for the human buttocks, at least from my perspective: "Ass-whipping" or "butt-whipping," "ass-thrashing," "seat-smacking," "rump-smacking," "bottom-smacking" or "butt-smacking," "fanny-tanning," "fanny-whacking" or "butt-whacking," and of course "bottom-blistering"--or even better, "bare-bottom blistering."
A "blistered boyish bare bottom," I'll take that from you anytime, ma'am!

--C.K.