On the contrary, the mechanic works best when there's consensus between DM and players about what it means. And, contrary to the table-fight horror stories that make their way to online alignment threads, consensus really is the norm. It's seldom contentious that the monsters trying to destroy the town are evil and the heroes trying to protect the town are good. We should keep it in perspective that hard corner-cases are the exception rather than the rule in this game. Personally, I've been playing for nearly two decades with a variety of different groups, and I've never had a serious disagreement over alignment in actual play. Ever. Seriously.
Thank the gods we have your vast and universally applicable experience to guide us and tell me how wrong I am about all the alignment arguments I've witnessed and been a part of since I started playing about 1980. Certainly you and your groups must represent the entirety of anyone's possible experience with D&D and I bow to your wisdom...
Oh wait, I don't. Look, alignment doesn't work
best with consensus about what it means...it
only works with consensus about what it means. Otherwise, we end up with the "orc baby" scenario or any of a hundred others that come up virtually every single time a DM tries to "enforce" alignment. Every. Single. Time. At least in my experience.
You don't want to reward characters for specific behaviors. That potentially leads to degenerate incentive structures -- e.g., "I get XP for donating valuable treasure to my church, so I should acquire as much valuable treasure as possible by any means necessary!" That doesn't seem like what you were going for, does it? Where alignment is concerned, general is better. If you want a character to be an honorable-knight sort of guy, you should have him behave with overall honorable-knight-style conduct, rather than picking out some particular actions you associate with honorable knights.
I can't agree that
general does anything to prevent degenerate behavior. I've witnessed far too much "Lawful Stupid" and other less-than-polite euphemisms for degenerate behavior, especially when Paladins or other morality-centric characters are concerned. I've also played other games that operate on similar premises, and they work fine.
But really, you don't want to reward characters for conduct at all. What if this character stops putting himself at risk to defend the innocent, and instead starts putting the innocent at risk to profit himself? Is that worse? Well, morally, yes, of course it is, but from the perspective of gameplay and narrative it seems like a perfectly valid character development. And it's not really fair that the character would keep reaping XP/inspiration awards for staying an honorable knight but must forgo them by changing conduct this way. Incentivizing LG/honorable-knight conduct (or whatever conduct matches the alignment/alignment-surrogate selected at character creation) inhibits natural character growth and evolution. Alignment is a tool for describing character conduct, not prescribing it. Acting the honorable knight? Wonderful: call 'em "LG", no penalty or reward except those that come from in-universe alignment-based effects. Acting the sanctimonious hypocrite? Just as wonderful: call 'em "LE", no penalty or reward except those that come from in-universe alignment-based effects.
Firstly, I was responding to the premise of the thread which is, in part, about "enforcement". That sorta requires prescribing behavior. Secondly, most of the games and alignment-replacement systems that actually carry out the sort of system I'm talking about allow or provide for changes in the selected behaviors (D&D is the system that didn't). In a D&D context, I've seen this opportunity provided at level changes and at a character's rejection of his alignment keys (See the
Sweet 20 Experience System). Thirdly, this kind of thing is already in 5e as the Inspiration mechanic deriving from the Ideals, Bonds, and Traits of a character's background. It is merely a very mild version. It doesn't seem to be destroying anyone's game. Fourthly, back in the day, you couldn't avoid this, because several classes had alignment/behavior restrictions that inherently enforced such restrictions and explicitly prescribed the results of losing your status as a paladin or whatever. So the question of "is it evil to kill a goblin baby"? could take on dire consequences for some characters and players.