I can absolutely enjoy fairly rules-heavy systems if those rules are relatively elegant and consistent, and mostly important, serve a clear and logically and design-wise justified reason for their existence. 3.5E/PF1 fails every single one of those tests. The rules aren't elegant. They're not consistent, and they actively unjustified by the design or by logic, and even genuinely offensive design ideas like intentional noob traps.
But why do I and indeed pretty much entire group at this point prefer lighter systems? and the -er is stressed here - we're not necessarily enamoured extreme rules-light systems, but PtbA, Spire/Heart, and Mothership have all been popular of late (and CoC, which is slightly heavier but not much).
1) Lighter systems tend to have rules that are much better designed re: creating the game the game SAYS that it is.
Sorry for the clumsy way of putting it, but I hope you get what I mean. My strong experience, I know my main group agrees, is that the heavier the game, the less likely it is the rules are in-tune with the vibe and concept of the game, and the more likely the rules exist for the sake of existing. I think MASKS is a good example, because it has some of the rules most perfectly harmonious with and promoting of the the concepts and conceits of the game/setting, and that heavier rules would eventually have started just getting in the way.
My main group has just really loved games that provide the feeling/vibe of the genre/setting they claim to be about since we started encountering them in the 1990s. And lighter games have consistently been better for that.
2) Lighter rules tend to be more responsive to approaching problems in various different ways, rather than just doing what's most mechanically effective/efficient.
Particularly it's much easier for the DM to just say "Ok, that works", often without even a roll, because in a lighter game, or to easily allow an idea to work, because the the rules aren't providing as much of a barrier between the players and the fiction of the setting.
3) Lighter rules tend to be both easier to learn, and yet often about as mechanically engaging as more complex systems, and they seem to gel well with the imaginations of the players.
This isn't always the case - Mothership I think is just borderline too light at times, with the DM (not me) basically having to make up some slightly more consistent rules than are actually provided, but for the rest, it is true, and even with Mothership, it works well with the imaginations of the players - people come up with good ideas more easily, in my experience, in lighter games, than in heavier ones.
4) There's a less of having to dismiss ideas/solutions which make either logical/conceptual or vibe sense because they don't make mechanical sense. I mean, I've kind of covered this already, but this matters a lot in my experience. In 3.5E, for example, we constantly had to reject ideas just because 3.5E didn't want that to work (the whole "a rule for everything" deal just really opposed creativity and even just logic at times).
5) There's an absolute metric ton less prep time for the DM. This is also really important and frankly has been so cool for my group because after playing PtbA games, one of my players realized, yeah, he could be a DM, and what was holding him back was the sheer amount of prep and rules-knowledge required (as he perceived it) to run something like D&D well. And now he's a really good DM for other stuff, particularly horror games.
6) Lighter games tend to let players contribute more to the fiction without putting them in the awkward and for some uncomfortable place that some very rules-light games do of having to actually wholesale create entire parts of the fiction. And I think that's kind of a "gateway drug" to DMing in ways.
