The rule could be anything, as funny or as serious as you want. The universe will progress in a similar way that it has up until this point, unless your changed rule prevented it from doing so.
Some examples might be:
- The invention of currency is not allowed.
- Iron is slightly less stable.
- The Ancient Greeks are able to cultivate Silphium, which does not go extinct now.
No suffering, no dis-pleasurable state, no undesirable reality exists. Everything that is, is deemed good by all beings that can judge it (if any). This has, as a consequence, no moral dilemmas, no conflict of wills and interests, no tragedies, etc.
What happens if I light my head on fire
I can think of two scenarios. The first one is you do that and everyone, including you, feels it and perceives it in a good way. Which I guess could end up in weird situations. Geeky example, but do you know Magic the Gathering? There’s a faction there called “The Rakdos Cult” with a demon and a lot of deranged characters that simply enjoy the bad things. The Rakdos cards often portray a little gorey scenes with people enjoying it, so I guess we could become kind of that but without victims, only enjoyers.
But the other scenario is that we wouldn’t have a need to prove or try such things because we often do it out of negative feelings such as emptiness, pride, competitiveness, etc. We wouldn’t feel those things so we wouldn’t behave as erratically as we do now.
That if we exist at all, though… Maybe existence as we know it is incompatible with my first comment.
Then all life would have likely gone extinct long before they even developed spinal columns.
Remember, pain is a fantasy your brain creates to keep you from injuring yourself.
Getting rid of that will mean zero survival instinct.