I'm going to list a few dos and don'ts as see how you feel about them. I'm a HYOH type but there are a few AT practices that everyone should know.
This list is not at all comprehensive. Feel free to add on or disagree:
DOs
Pleasantly nod and briefly greet hikers you pass on the trail - - especially in remote areas
Share important trail info (i.e. - "I just saw a bear at ___." or "There is no water at ___ shelter." etc.
Yield in general to uphill hikers (unless special circumstances exist)
Step to the high side of the trail to avoid erosion
Keep generally on the quiet side while hiking and camping so others can enjoy
Practice basic LNT - - you should adhere to at least some forest preservation and care (if you want to call it something else, fine)
If you are stealth camping, no one should be able to tell you were there at all - no fire ring, no tent indention, nothing.
Offer to get water if you are going to the water source - be helpful - offer hungry thru-hikers extra food if you are a weekender with plenty
Hang your food or bear-box if you are in the shelter - - if you are tenting, do what you do.
Throw mulch and leaves in the privy
Be respectful of the trail, the forest, wildlife, and other hikers.
Don'ts
Don't try to chat-up everyone you see - a nod and a smile is usually plenty
Don't get involved in someone else's epic (unless they are in real trouble) - fine line here.
Don't build a campfire just because you are camping and don't build new fire rings (try a candle - they are nice)
Don't build fires in the wind and cut down things in the woods
Don't smoke in trail shelters - - some folks are allergic - - just politely step outside
Don't pitch your tent in a shelter (unless it's the dead of winter and no one is around) - it's rude
Don't make a bunch of noise - people are mostly out for solace and relaxation - - they probably don't like your classic rock or whatever
Don't expect people to fetch you water and give you food - - asking and offering are a big difference
If you know you are going to fart and snore all night long or make an ass of yourself, you should tent.
Don't leave your toilet paper on the ground - - it takes 2 seconds to bury it and it takes 2 months to go away if you don't.
Don't pee in the privy (unless it just comes out and you can't help it)