Thanks, I'd seen these, but didn't really understand them; your message made me take another look to them. I think it wouldn't take more than five minutes to document these parameters better, and to add a bit more info to the section "User documentation" of this page, since it doesn't even say how to make LQT either opt-in or opt-out ("comprehensive documentation" may come later, but at this point it is so scarce as to be almost useless). So to save time to other people who might have the same question, this is what I did to make it opt-in: removing it from talk pages by default, leaving user control true (so that users can always turn them on if they wish).
/* Allows switching LiquidThreads off for regular talk pages
(intended for testing and transition) */
$wgLqtTalkPages = false;
/** Whether or not to allow users to activate/deactivate LiquidThreads per-page */
$wgLiquidThreadsAllowUserControl = true;
To make it opt-out, i.e. to use it by default in all talk pages unless someone turns it off, just don't touch anything.
The reason I chose opt-in is reading about various bugs in various places (eg. in Wikieducator), which doesn't make me confident about deploying it sitewide, and the fact that this seems to be current practice in various wikisites: add LQT in selected places, but not on all talk pages. By the way, there are also a couple of settings that I don't understand:
$wgPageProps['use-liquid-threads'] = 'Whether or not the page is using LiquidThreads';
/* Allows activation of LiquidThreads on individual pages */
$wgLqtPages = array();
What are valid values for $wgPageProps['use-liquid-threads']? And for $wgLqtPages? I tried putting a page title in the array so that only that page would use LQT, without any result.
And, finally, what's the role of those tags mentioned in one of the replies? Where does one use them?