What was the single best decision you ever made for your forum or website?

Running a website or forum comes with endless decisions, but usually, a single choice can turn out to be a total game-changer for a community's growth.

Looking around at successful platforms, decisions like switching to a clean mobile design, trimming down unnecessary subforums, or simplifying the registration process seem to boost activity a lot.

If you own or manage a site, what was the single best decision you ever made for your forum or community?