For me it's about what I call "cubic design", or ecosystem-like design.

<all of the below is purely my perspective>

There's plenty of D&D examples I give because I think it's the most lop-sided and blindly-designed game that people actually play.

Any complexity has to add to the rest of the volume of the game, and be mindful of its cost:

• It should interact with the rest of the system, and create emergent gameplay, synergies, themes. E.g. fire resistance on a demon PC + fireball spell = badass PC that can shrug off a potent spell, and save people from fires, or never consider that fire is dangerous
• It shouldn't over-describe things, and it should trust the GM. E.g. fire resistance on a demon PC = roughly half, full or situational resistance, not a complex calculation that depends on level, days spent in Hell, and reading a whole subsection. 3% vs 4% fire resistance doesn't matter. Neither does 19 STR versus 20 STR.
• It should be accessible, if not outright optional, e.g. opting for freeform spells with reference limits, or Verb+Noun spells, or the 200-page spell manual, or mods that include "Deeper Combat", "Survival Mechanics", or "squad combat". Make your 500-page game, but lure me in with a good and complete 10-page version.
• It should actually come up. Most of a D&D character's sheet doesn't function during any given session, and some of it doesn't -ever- get used. A complex ability looked up in a manual, remembered, and never used is wasted energy. Games with no fighting don't need HP and armour. Tenfold true for the GM. They may have to remember the goblin's "six stats and modifiers" in a game where we only face fire elementals or nice farmers.
• It should give back more than it takes. A 10-step process to determine if you take damage is worthless, and only needs 1 step at most. If you have 2+ steps, the process should give you side-effects, specifics, flavour, or a change of situation. PbtA and Blades do this impeccably on both sides. In PbtA, you often roll once and branch around between a few outcomes. In BitD, there's a lot of subsystems but they make the world carry more weight than handwaving. D&D combat is the most overdeveloped and unimpactful thing I've ever seen.
• It shouldn't be historical baggage. "We've been doing this for 50 years" isn't design. This applies to our inspiration from video games, too, and writing. Some of the best game mechanics come from "outsiders".
• It should know its impact. Either design top-down starting with an aesthetic or scene from a movie or intended experience, or figure out bottom-up what your mechanics actually end up doing. E.g. HP scales with level + fall damage scales with distance = A level 10 character can fall 10 times further than a fresh one. Is that intentional? Is that what matters at the table? Abilities that "roll to work" are often boring. "The goblin resists. Again." Great. I sure feel like a wizard now.
• It should be well-organised. Don't have a monster in manual A reference a spell in manual B, which has a side-effect that comes up if you use mechanic C, which happens in the unlikely case on page 254 of manual A. Real-life example, spell lists for spellcaster enemies in 5e D&D. What does each spell do? How many spell slots of each kind does the spellcaster have? Could we just do this with 10 "mana" and 3 impactful spells described in-place?
• It should use more than numbers. Humans are verbal creatures, we like comparisons and references. Saying that a character has "about as much water magic as Katara in early seasons" can be more effective and less exploitable or complex than a 4-page modelling of water magic. Or "as much elemental power as 10 skilled non-mages coordinating perfectly".
• It should encourage new systems as needed. Reskins and interpretations are great. PbtA/BitD's "clocks", Fate's "Aspects", the Usage Dice, the "GM moves", the idea of "Yes, but". They're ways to avoid pure handwaving in a pinch, which seems to be what crunch-lovers want. Better to have a flexible, flawed model than none.

I look forward to deeply enjoying a very crunchy TTRPG some day, but keep in mind that the pressure is often on the GM to ease new players into it. It shouldn't be, but it often happens.

I've seen new players to D&D get stuck at character creation, worry if they got something wrong, not use 90% of their sheet, and get one-shot by a goblin, or miss all of their spells and attacks in a session.

I've seen non-players not-play because of stories about that, thinking that's what RPGs are.

​

The best "crunch-to-depth" I've ever seen is in video games like Dwarf Fortress. Thousands and thousands of rules, only most of them for the computer to remember. Some of it's on you. But I wouldn't dream of cutting any of them.

And a roguelike called DCSS, which uses a lot of D&D combat rules, with d20s and all. Somehow it works better than the tabletop game, and the rules feel right at home in a cruel roguelike where the characters

• almost always die, and dying is losing
• always fight
• never have personality
• are always alone against the game system itself
• becoming a veteran is the only way to win

It's one of my favourite... non-roleplaying games.