The Druid argument is one example of current gaming attitudes that frustrates me somewhat, namely the unwillingness of players to abide by certain core themes of classes and races in their roleplaying.
When I DM I'm of the opinion that if you play a Druid, your character will NOT wear metal armour. This is not a player choice, this is just a simple rule that a player must abide by. If the player is unwilling to do so, then they will not be playing a Druid. It's really that simple. If a Druid player tries to get their character to don metal armour I will simply veto the action and say 'No'.
So the problem with this is two fold. First, starting with 1e any druid could put on metal armor. It's why even though 1e through 3e used words like "can't" and "prohibited," they also informed the players and DM that metal armor kept the druid from casting his spells. You could in fact put it on from the get go, even with the restrictions. The second problem is that 5e has removed the penalties and the word "can't," so that now it's just a fluff preference of the class.
You can of course reinstate the penalties for wearing metal armor that were created by the old school, but if you actually tell the player "No, your PC doesn't put on that armor," not only are you committing one of the cardinal sins of DMing, that of controlling a players PC without in game means, but you are also doing something that was incorrect as far back as 1e. Druids have always been able to wear metal armor.
1e: The more powerful druidic spells, as well as their wider range of weaponry, make up for the fact that druids are unable to use any armor or shields other than leather armor and wooden shields
(metallic armor spoils their magical powers).
2e: 2e does not have the same language as 1e and 3e. It can be assumed, though, to be like 1e where it was just a restriction due to prevention of spellcasting and abilities. All it says is that metal armor is forbidden, not why.
3e: A druid who wears prohibited armor or carries a prohibited shield is unable to cast druid spells or use any of her supernatural or spell-like class abilities while doing so and for 24 hours thereafter.
4e: I have no idea. I didn't really play it much and druids aren't in the PHB.
5e: Makes wearing metal armor a personal choice by deliberately and repeatedly stepping back from prior language like "can't, "forbidden" and "restricted," using "will not" instead. It also removes all mechanical penalty, further reinforcing that it's just a fluff choice not to wear armor, which any PC can of course change.
However I'm somewhat old school in my approach to other classes and races, I use the racial preference table from 1E for example - sorry, if you want to play a Half Orc then the Elf in the party is NOT going to be a lifelong best friend. In fact your characters are not going to get along, at least not initially, so please work together and decide which of you is going to play a different race that is more suitable, or be prepared to roleplay significant conflict between them.
As was noted by [MENTION=6999115]Ohmyn[/MENTION], PCs are exceptions to the general rules. They are free to pick and choose their personalities, including have compassion or understanding for races that had animosity for one another in the books. This is a common trope in fantasy writing, and is backed up by D&D itself. Official products have had good demons and other exceptions to the general behavior rules, because individuals are.......individuals. They can decide for themselves if as an elf, they hate orcs. Maybe this one elf over here wants to heal the divide and thinks orcs can be redeemed. If you as the DM are forbidding players from doing that sort of thing, not only are you stepping over the line with regard to playing their PCs, but you are missing out on tons of great roleplaying opportunities.