Three Pillars

A much shorter one this time.

When developing 5e, WotC very wisely identified three pillars to the game: combat, interaction, and exploration. These three are probably not the only things in the game, but they make for a remarkably solid cross-section. And a game that is strong in all three is very likely to be a very strong game, while the weaker any of the pillars are, the less it's likely to appeal to some players.

I'm inclined to suggest one further thing: every player character should be able to meaningfully contribute to situations involving any of the three pillars.

(Amongst other things, that probably means that tying interaction to a single stat (Cha) that's of value to only some few character classes is a really bad idea. That creates a situation where the party Bard/Sorcerer automatically becomes the party 'face' and gets to enjoy all of that pillar while everyone else sits back and watches. But, other than simply removing the social skills and divorcing Charisma from interactions almost entirely, I'm not sure what the fix would be. Or maybe that is the fix!)