I'm very defensive about my changes but:
1) No monks. I just detest the idea that two people could spend the same amount of time trying to learn to fight, only one uses their bare hands and the other an actual weapon, and they end up with the same result. It's nota balance issue. It's not even a setting issue really, as I could fit monks in. I just am annoyed that this 'Kung Fu' class ever became a thing.
2) No hobbits. This isn't middle earth people. I love the good Professor as much as the next guy (or more), but stop being derivative.
3) My falling rules are a picture of baroque complexity because at the same time I'm trying to be a) lethal, so people don't shrug off falling, and b) nonlethal, so people can shrug off falling. It requires a good page or two to even describe how they work, so I won't here unless someone is interested.
4) "Chainmail": It's just 'mail'. And it's plate armor, not "plate mail". And 'studded leather' got thrown right off the table, darn it. Right now the bet is on how long I can go before I finally change the 'longsword' entry to 'arming sword', and convert 'bastard sword' to 'longsword'.
5) Oh, and my 5' step works 'backwards' of the raw, because you shouldn't be able to just step back and fire off an arrow while a guy is trying to poke you with a sharpened metal rod. So you can step in to a fight, but not out of it.
6) When a stirge lands on something it is not "grappling" with the target. It is "clinching" the target. The target may want to initiate a grapple to break the clinch, but the stirge has absolutely no interest in controlling the motion of the target. It just wants to hold on.
I could probably go on and on in this vein. I'm the king of making tons of fiddly little changes because some tiny detail of the rules annoys me.
Sounds like me. I habitually rewrite each edition's rules to tweak out things I don't like and fit my campaign better.
1) No monks right now (or druids, warlocks or barbarians for that matter right now, just NPCs). No dragonborn or tieflings either.
2) I'm very derivative, so you wouldn't like my Realms...it's what I like.
3) Mine are pretty simple, because they work off of my injury rules. That is traps and falling cause injuries (which take at least 3 days, usually longer, often much longer to heal. Uses the exhaustion track.). Happy to share as well if anybody is interested.
4) Agrees, I have padded jacks, brigandine jacks, mail, scale mail, etc. Longsword is actually a proper sword that is longer than a bastard sword, which is longer than an arming sword. That is, an arming sword has a one-handed hilt and is longer than a short sword, a bastard sword has a longer hilt primarily for balance, but could be used two-handed (usually called hand-and-a-half because of the length of the hilt), and long swords have a full two-handed hilt, but could be used one or two handed, unlike the zweihander.
5) I don't use initiative, so generally speaking, if you're in melee and attempt to step back you're usually followed. Attempting to fire an arrow whenever somebody is within closing distance (30 feet for me) is not a great idea, unless your shot happens to kill.
6) Yes.
Most of my rules start with a pet peeve about either the world, or a scene that I'd like to model but can't under the current rules. Starting with "I am not left-handed." Others are modifications to the healing magic to eliminate the meaningless scenes where somebody is too injured to help, but for some reason you can't just cast a healing spell on them. This also applies to poison and disease. My crafting and learning spells rules take a long time, because I think they should. I'm designing and running game to model a world, not add more cool benefits and abilities for characters.
Oh, teleportation or flashy abilities that theoretically don't cause a problem with the game. The teleport 30 feet thing seems to be a favorite of the designers right now. Sure, it's not any more powerful than moving 30 feet, but I don't like the idea that x% of the population now teleports, or flies 10 feet when they cast a spell, or whatever. Same thing with the disappearing weapons. If you have a magic item or a spell, fine, but just an innate ability that every fighter or warlock of the correct class has? Completely changes the world in ways that I don't like.
The other big difference is the rate of advancement. According to the Adventuring Day on pg 84 in the DMG, they go from 1st to 2nd level in a day, 2nd to 3rd the next day, two more days to reach 4th level, by the end of the month, you're 17th level. By day 35 you're 20th level. Even playing the APs you're expected to gain around 15 levels in what takes a matter of weeks, maybe a couple of months of in-world time.