Like meetings, you need a firm deadline to ensure a decision gets made–it just needs to be a little more flexible than "right now". Like the free-for-all conundrum, you need to have space for different ways of thinking (bullets, comment threads, screenshots)–it just needs to be housed in one place. And like random threads, you need your decisions to be made in a tool your team already uses every day, not yet another software to slap on top of your stack.