Compromise between
mobility and
protection,
firepower being an uncompromisable requisite.
That's my version for the time being, swap around the word in bold in the sentence above to your liking
.
Also, no one ever thinks about this (except tankers, I guess), but reliability, maintainability, general sturdiness and affordability are musts. You don't want your mother-of-all-tanks to fall apart ten meters after rolling out of the assembly line...