I have about 54 years experience with firearms and about 10 days experience with ARs. As I go through my learning curve, I thought I'd document some things I learn along the way. Just in case they might be helpful to others in the same boat. This isn't about big stuff like design or best ammo or anything like that. Just stuff I'm learning as I go along.

1. Test fit all accessories at the same time. Don't install one, then another, and etc. You will inevitably have to remove one or more of them when you go to fit another accessory.
2. M-LOK rocks, in my rookie opinion. Pay attention to what's on the other side of the fore end when test-fitting things. My first flashlight position wouldn't work, for example, because the gas tube was there and there wasn't enough room.
3. Try not to drop screws, nuts, Allen wrenches, and etc. inside the fore end.
4. Gun vise grips work a lot better on flat, wooden stocks than they do on irregular collapsible stocks. They still work, they just take more futzing.
5. Pay attention to fastener heads; accessory manufacturers seem to use hex and star interchangeably with no rhyme or reason.

Added:
6. A set of good tweezers comes in handy.
7. Hold off mounting accessories until a) you know you want them and b) you've studied up on how they are used, their benefits, their drawbacks, the situations in which they are useful; those in which they aren't, and etc. I installed all of mine before I learned this from the members here. And they are right. I've spent a fair bit of time removing and moving accessories.
8. Test-fit accessories and operate everything else on the rifle to make sure you know how everything else is affected. Safety, trigger, stock extension, bolt catch, whatever.
9. Don't mount your cool new optics backward and then post a pic of it, like I did.
10. When someone tells you your optics are mounted too far back, listen to them (I didn't).
11. Use a large plastic bag whenever you're dealing with springs that have to be compressed to remove or install something. I learned this here and it's already saved me a half-dozen times.

12. To be continued as I learn more from my fumbling around.

The biggest curve you'll find is learning Chinese/Air Soft parts being sold and American companies that sell quality, but still good price. Chinese have gotten good with their fakes.

There is also the difference of "commercial" made size and "mil-spec" size like the Extension tube and difference of Weaver and Picatinny rail.

You'll also see and learn about what lubricants to use that will boggle the mind.

Does one dimple the barrel, stake the extension tube nut, chrome-lined or cryofreeze of a barrel, 1:7, 1:8 or 1:9 twist and so much more.

Welcome to Lego-land.

So much to learn. Thanks!

99% of what you just put on, you'll take off because it is nonsense and gets in the way.
Brand fanboys have malfunctions too!

Another big issue is paying good money for a specific config and then replacing everything.

I always suggest starting with a basic AR and then after putting 500 rounds thru it change only what needs "fixing" based on your actual experience.

I'm a bit guilty of that but I came to the game late and started out chasing a state-wide ban.

Top post updated.

I'm in the same boat as the op. I have 40+ years of trigger time behind the M1 rifle and know it intimately inside and out, but have only about a year and a half behind the AR 15. My saving grace is I like simple. I've installed a 2-stage trigger in it, added an optic, and an M1907 sling for NRA/CMP xtc matches. It's as "accessorized" as it will ever be.
KISS, gentlemen.

Jon

#1 lesson I've learned over the years? Less is more. A light, reliable rifle with a clear optic that has a good field of view and eye box is WAY more useful than most other setups. The others have their place, but are usually trying too hard or fit very specific use cases well and everything else poorly.

I swear, the older I get the more I think I should just go back to a lever action 30-30 and call it a day for most needs.