Problem: the clerics and mages of the party want to use swords and bows.
Solution I: eliminate the rule that bars the use of weapons, so that the new rule is that anyone can use any weapon.
Effect: momentary happiness, followed by all characters quickly adopting only the most obviously useful weapons. This then results in a thin greyness of weapons use, adding to the ever growing ennui as every nuance of game play is steadily shellacked over until it is impossible to distinguish one characteristic from another. Characters around the table eventually adopt the habit of dropping character names, replacing them with numbers, making it easier for the DM to remember that the first player on the left is always referred to as "number 1."
Less humorously, providing the players with easy access to whatever they want may temporarily please the DM's group, but it does little to add to the game experience, which relies upon the players feeling that they've produced a sort of achievement through innovation. Answers like Solution I dispel the need for innovation, by simply handwaving the obstacle away. There are dozens of examples like this that have been emerging in later editions, which some will insist are "advancements" but which are, instead, a call for conformity. Without the need to innovate to overcome obstacles, we get the sort of rhetoric that argues that "rulings," rather than "rules," can produce innovation ... except that most rulings fall under the heading of solution I, which kills innovation to get around problems.
Another word for this conformity is the call for game "balance," where every player is given the same opportunities and privileges ... but again, that only eliminates the need for a player to find the quality in some character drawback and ~ through innovation ~ prove that it actually is an opportunity for gaining advantage. I point the reader to the following video; I may have linked it before, but watch it again anyway.
But innovation isn't easy. And most players will sit and pout and complain about limitations because they can't be motivated to innovate past them ~ they want and they want but they don't work, and they don't think that they should. In fact, from the players point of view, rulings implies a possibility of massaging the DM into making a decision that will bypass the need to innovate, while rules put a bullet in the DM's gun when the player is being told, "No, I'm not giving you something for free."
Solution II: find a pathway that circumvents a hard rule and make the pathway a new hard rule. Ensure the pathway is possible and perhaps not even hard to accomplish, but ensure it is time consuming and contains elements of hard obstructions.
Effect: some players will advantage the rule, though not all. Some players will despise the rule, but not all. Some players will complain and rail as they go through the motions of the rule, eventually emerging on the other side with success that they will, at first, view with suspicion. And then, steadily, they will begin to comprehend two things.
First, that they earned this change. At first, it will simply seem like a bunch of needless hoop jumping that could be waved away by the DM, but once they've accumulated the time they will see themselves as entitled to that change whereas others are not. This will mean that others can't just co-opt what they have earned and they ~ the players ~ will argue very strongly that the rule should not be hand-waved.
Second, the players will become more conscious that the mage or the cleric they have now, that is able to use a sword, will not be as easily replaceable as mages and clerics once were. If the character dies, yes, they'll be able to go through the hoops again ~ but it will require going through the hoops again, which means a clear understanding that a character is not just what we roll up at the start of the game, but is something we obtain through work.
I cannot remotely begin to express the importance of this. A huge failing of 5e is that no matter how much time you put into your character, it doesn't feel like you're achieving anything of real importance. Another player joins the game and is automatically given what you've just taken a year to accumulate. Your character dies and you're automatically given what you took a year to create the first time. At no time do you feel that anything matters, not to you, not to the other players, and certainly not to the DM. There are no pathways that you can take in the game that give you any sense of real acquisition.
Because treasure, magic items, extra skills, even levels ~ these things are not "real." They are elements of the game. Referring to the video above, if your fox character dies, you can have another fox. The only thing that makes it "real" is the creation of the tournament, which means you're losing ~ and something more meaningful than just another set. The tournament becomes the achievement. Playing the game without keeping track of your wins doesn't provide enough meat to keep playing. You've got to play towards a purpose, towards something that you can feel passionate about ... or else you really are just going through the emotions. And however much fun that was at the start, after a while, it's not fun any more.
Players create all sorts of side games to give a particular game session meaning to them. It ceases to be about the character and what the character can do, or the achievement of the quest, or even the storyline of the DM's imagination. More and more it becomes the jokes that you can tell, or the opportunities to prove you're clever with a phrase that will humiliate a fellow player or make the DM blush. After a while, you're settling in to take advantage that there are four or five people contained inside a room doing anything, so that you can be sharp-witted as you jump your brain ahead of your neighbour in making the best possible joke in the least amount of time. The jokes are real. The jokes aren't pretend events, they're actual demonstrations of your personal ability to innovate successfully.
This is what the game is meant to provide. But if the game won't, then real life, and real human relations, will step up.
The rules of D&D have to be more than just boundaries and barriers. They have to be building blocks and pathways and full of complex perspectives that, employed just so, will produce a new combination of tactics that one player, and only one player, will invent on the spur of the moment. Which, in turn, the DM has to allow because the rules provide the player the power to say, "I'm doing this, and you can't fairly make a ruling that stops me."
This, I know, scares a lot of DMs. But me, I love it. I absolutely love it.
"I just hope you see how it's even possible for something so weird and seemingly innocuous to inspire so much passion. How a party game that's basically the [pretend game] equivalent of a twelve-year-old smashing their toys together can be fun, hilarious, freakishly complex and, from the right vantage point, even ... beautiful."~ Ian Danskin, Innuendo Studios