How you double damage depends on the effect’s delivery. If the damage comes from an effect with an attack roll, you roll double the normal number of dice and apply all damage modifiers, bonuses, and penalties twice. If the damage comes from an effect allowing a creature to attempt a saving throw, you double the damage total instead, rather than rolling more dice.
I know it's a relatively minor thing, but why the distinction? As one of the goals is streamlining, why not just use one method?
I was confused by this, too. I would definitely vote "always roll double the dice" because rolling lots of dice is fun.
If you cast a fireball, you don't want 2 different throws for those who failed and those who critically failed
I too thought this was a weird distinction, however in the case of multiple outcomes for spells that hit multiple targets, it now makes complete sense.
Honestly, if it weren't for things like Deadly adding bonus dice that don't get doubled on crits, I'd just say roll once and double the total for both cases to save time. And dice.
You could potentially wind up rolling 12d12 for a crit with a +5 greatsword. That's more d12s than I've ever felt the need to own.