The creators of Dungeons & Dragons introduced some rules that don't make sense, but they should be used by a DM anyway, as suspension of disbelief is sometimes needed to make the game as fun as possible. The rules in the D&D 5e Player's Handbook aren't intended to accurately reflect reality: they're intended to be the framework for a fun fantasy game. D&D's massive global popularity proves that it does the trick, even if the players need to accept that things won't always make sense.
The reality of the D&D game exists to support the game, which is why everyone walks in increments of 5ft and why rounds of combat last around six seconds. The restrictions will, inevitably, lead to arguments among the DM and the players. These will sometimes manifest in ridiculous demonstrations, with players measuring out 5ft steps, while trying to argue how their character could have totally avoided that attack of opportunity.
D&D 5e has far fewer rules exploits than the older editions of the game, as D&D players used to cheat through loopholes all of the time. Nowadays, players are quick to tell the developers about exploits online, so they can publish an official errata to fix it. It also helps that the developers had nearly fifty years' worth of player data to look at when crafting D&D 5e. This isn't to say that everything is perfect about D&D 5e, as there are still elements of the game that don't make sense, but they exist for the sake of game balance, or to stop things from becoming too frustrating.
Spellcasters in D&D can do some amazing things. All it takes is a single spell to travel between dimensions, transform into a dragon, or unleash a lightning storm on an enemy army. The D&D wish spell can even rewrite reality itself, but that's a power held by only the most powerful of arcane spellcasters. This power comes with some restrictions, as almost every spell in D&D requires the player to be able to speak and to move at least one hand. This is actually more forgiving than in previous editions, where the caster needed both hands to use magic.
The D&D 5e Player's Handbook has a neat section that lists all of the status conditions in the game. These are status effects that can debilitate the player or their enemies, without necessarily dealing damage. Some of the status conditions are surprisingly lenient, as neither the Grappled or the Restrained condition prevents someone from casting a spell. It was later clarified that a D&D character that had hands bound while Restrained couldn't cast spells, which gives the DM the room to penalize a player if the situation calls for it.
The reason these status conditions are so lenient is that they would completely shut down spellcasters if they weren't. A single web spell, a net thrown under the cover of darkness, or an orc that gets the drop on a caster and throws a bear hug on them would rob a character of their main ability in combat. These looser restrictions mean that a caster has a chance to respond to these threats and isn't one basic action away from being robbed of their spells. It just makes things less frustrating for players.
It's fairly easy for a D&D character to possess the power of flight. There are some races that can even naturally fly, with D&D's fairy and owlin giving players a flying speed. A spellcaster with the fly spell can also take flight without the need for wings. The problem with introducing flight rules is that the game also needs falling rules. All it takes is one dispel magic to send a floating wizard crashing to the ground, so the game needs a framework for how to deal with the damage they take.
The D&D 5e Player's Handbook does have rules for falling damage. The character takes 1d6 bludgeoning damage per 10ft they fall, for a maximum of 20d6. This means any damage taken after 200ft doesn't count. What makes this so unusual is that 20d6 is more survivable than it seems, especially for a barbarian, which has the highest Hit Dice of any class (1d12) and their Rage ability gives them resistance to bludgeoning damage. A low damage roll from falling damage means a mid-level barbarian can survive a drop from orbit.
The reason this rule exists is to stop players from abusing it for their ends. If there wasn't a cap on fall damage, then high-level parties would start forming tactics around dropping enemies from a great height. It puts a hard limit on D&D player attempting a rules loophole, as they are likely to do, and it shuts down a lot of arguments, even if it means every character has the fall resistance of Omar from The Wire.
In D&D 3e, there was a whole set of mechanics based around performing a counterspell, where a player could use dispel magic to cause an opponent's spell to fizzle as soon it was cast. The problem with this tactic is that it required the player to have the higher initiative and to ready their action, then they needed to pass a skill check in order to counter the spell. Most players would rather just strike first than rely on a counterspell. D&D 5e scrapped the mechanic and replaced with counterspell, a third level spell that could be cast as a reaction and automatically countered a spell of the same level or lower, with the player able to expend a higher spell slot to empower its effect.
There is a bizarre rule involving counterspell. It's possible in D&D for counterspell to counter counterspell. The chain of actions works like this: Spellcaster A casts fireball, Spellcaster B uses their reaction to cast counterspell to stop the fireball, Spellcaster A then uses their reaction to use counterspell to stop the other counterspell, then continues casting fireball. This means casters can stop mid-spell to cast another spell in reaction to another spell.
This rule is actually a good thing, as counterspell is an incredibly potent tool when fighting spellcasters, especially as it only requires a reaction to cast. A sole party member could keep burning their reactions and spell slots to shut down a powerful enemy wizard, leaving their allies to gang up on them, or give them breathing room to deal with any minions kicking around. The counterspell response means that the biggest encounter in a Dungeons & Dragons campaign isn't neutered by one tactic.
