Got a whole bunch of ones that are political and/or religious (especially and) that I'm obviously not even going to hint at here. Just establishing a baseline for my personality.
D&D alignment is incoherent garbage and the only thing that can fix it is to discard it. All nine alignments as described in the AD&D PHB are dangerous mental illnesses, and in 3rd Edition they are all still too vague and subjective to be used as the basis for determining (or denying) class abilities.
For every sanctimonious nitwit who says that a Chaotic Good Paladin oath/archetype will "dilute the class", there are three officially published LN or outright LE options that they don't have a problem with.
Don't @ me on this. Just Do. Not.
Casting [evil] spells is an Evil act. It does not matter who/what you're casting [evil] spells on, it does not matter why you are casting [evil] spells, and even the consequences of casting [evil] spells do not matter. Casting [evil] spells is an Evil act and it will eventually (or even sooner) make you an Evil character no matter what other good deeds you do.
This also applies to creating the undead, including voluntarily becoming undead yourself, and it may even apply to involuntarily being undead.
The rules are clear on this and they make sense even if they don't coincide with most players' moral logic. If you can not agree with this, or at least accept it, you agree with me that the alignment system is garbage and you need to stop defending it. Especially from me.
As mechanical abstractions in combat, Armor Class and Damage Reduction are equally unrealistic and equally silly if you drill any deeper than the surface. Feel free to use either in your games-- I use both in D&D-- but please stop pretending that you're making the game more realistic or that you're even remotely qualified to do so.
Related... D&D was more realistic when all weapons did d6 damage. Differentiating them by damage dice was neat for mechanical diversity, but doesn't actually reflect anything about armed combat accurately.
Primitive black powder firearms shouldn't ignore armor, or at least not any better than certain other weapons do. The word "bulletproof" was coined about standard D&D plate armor, and claiming that your armor (as an armorer) was bulletproof legally required you to prove it. The English longbow and the heavy crossbow both had similarly nasty reputations for puncturing plate armor, supported by historical records of late Medieval knights petitioning the Church to prohibit them because they allowed mere commoners to kill them.
Modern kevlar body armors aren't better at stopping even modern ballistics than plate. They're lighter and cheaper and thus easier to equip for modern military and law enforcement personnel.
And the Sioux in the 19th century had traditional leather shields that unless a bullet hit them just right, that bullet would glance off harmlessly. Of course, if they did hit them just right, they went straight through and killed the guy carrying it.
Hate to say it, but you know what actually reflects ballistic weapons versus archaic (and modern) body armor pretty well, all things considered? Armor providing a bonus to Armor Class, with no special dispensation for the penetrative power of boomsticks.
The multiclassing system in 3.X D&D was broken on a fundamental level that seven years of feats and PrCs could only partially compensate for. Bringing it back in Fifth Edition only made it worse. That said, I have only ever played one single-classed character in thirty years, and I have no intentions of starting now.
Reskinning and dissociated mechanics are the bane of modern D&D. Separating the game mechanics from the in-game reality that they represent doesn't promote narrative freedom, it deprives both the game rules and the game narrative of any sense of meaning.