Well consistency would be nice. If alignment is a tool for judging character behaviour, then it would be nice if every example of actually using alignment to judge behaviour wasn't swept aside as an example of bad dming.
Can someone give an example of using mechanical alignment to judge character behaviour that is acceptable to the pro-alignment crowd?
I don't see Alignment as judging player behavior, but simply a statistic that represents the current behavior of a game element, a construct within the game.
Everything in D&D has an Alignment. Most everything in D&D has lots of the other common D&D statistics too (strength, intelligence, hit points, saving throws scores, and so on). All of these scores can change depending upon what happens in the game. That's a good thing. Same with alignment. It determines certain behaviors and can change depending on results.
Scores in relation to game mechanics tell the referee how any particular game element interacts with other game elements. That means Alignments when dealing with the Alignment system.
Player Characters as game constructs haven Alignments too. They also have scores like wisdom, intelligence, charisma, and slew of other stats, but only in so much as what a Player does not play in the game.
All the things a player actually does in place of a PC must be determined by the player. That means things like Morale, how much they remember of their past, what strategies they've developed, their loyalty to other characters (not just players), and so on. Everything in the Alignment system that is not defined for a PC, but is for an NPC or other game element, must be done by a player.
Player actions change their character. That's every score and resource, not just Alignment. When players take actions that would shift their Alignment score, then they are informed of that. I include a Saving Throw on an Alignment shifting action so they are informed before they even take the action. This way they can feel out the edges of where their current alignment stands in particular situations within the game.
Players do not need to play to a particular alignment just like no one ever needs to play Ability Scores as attributes. But just like any score in the game Alignment affects how other elements in the game will interact with their game construct, the PC.
Some classes are more difficult to play because they lose abilities when their Alignment shifts. The Alignment system
is the core system for the cleric class to master and get XP within (otherwise it becomes a poor substitute for gish class). And clerics who change Alignment often change significantly when that happens.
Cleric subclasses (and other subclasses like Rangers and Paladins tied heavily to the cleric sphere of play) are usually the Alignment-specific classes. Stop being Neutral and your Druid becomes a standard Cleric losing all their specialty Druidic abilities. Like any Ranger or Paladin who becomes a Fighter the Druid becomes the same level of Cleric and may find more clerical abilities in another religion/deity.
Paladins are a special case, yes. If they shift Alignment, they can never rejoin the ranks of Paladins in that campaign again. (Or you could just roll up another Paladin) But that challenge is what the Paladin class offers to players of it. Other Alignment-specific classes don't have this limitation.
Alignment in D&D is based off the three basic ways of playing a game. Cooperatively (together), Competitively (at odds), and Solo (each their own way). D&D is a cooperative game because the balance of the game tilts towards cooperation as the effective means of achieving mastery of the game for any individual. This mastery must still come individually for a player (not a character power), but the capabilities of a group as a team counts largely too.
Cooperation is not a rule of the game players must solely follow. It is defined as certain operations within the game where game elements work in conjunction with each other rather than, well, the other two alignment behaviors.
D&D is unique as a cooperative game (actually, it's incredibly unique in many ways) as it offers multiple systems for player to explore, each one selected prior to play according to the class they pick to focus on. Combat system. Magic system. The alignment system is one of the core designs of the game. What D&D does is make play of these roles complementary to each other with both niches and overlaps within the game construct which is the fantasy game world. Most cooperative games, and even 20 years ago there were few, make every game piece the same with only varying stats. However, this last difference is what makes D&D a role playing game.
So, before playing the game Players select an Alignment to start their PC in. They play the game and learn what those actions mean in relation to the Alignment system, or ignore it if they wish, and go about their business of achieving objectives within the game. But the DM still tracks their Alignment. Just as the DM tracks the changes to their age, location, what equipment they have, what scores their PCs have, and all the rest. These will change and its up to them to tell the Players when they do.
While hopefully this generally covers character behavior and alignment it doesn't really get into judging Players and how they behave. A referee running a game of Monopoly, a DM running D&D isn't judging the players as good or bad people, but rather keeping track of how their PCs' scores change throughout the game. That's not ever bad DMing IMO, but necessary to even be a referee.