I think that proper calcium and alkalinity levels, coupled with parameters on par with a typical reef tank are critical. We've had moderately good success growing zoanthids, but not even week after we started supplimenting (after a year and a half of NOT supplimenting) for calcium and alkalinity, several frags are exploding with growth. Things we've had for months, and some things we've had for years are showing growth like they never have. Its amazing to come home after a week of being up at school to see 2-4 new polyps growing on a frag of zoas, or a colony suddenly bursting at the seams with polyps.
But lighting is important too. Basically, individual factors matter, but all collectively make the biggest difference.