PREACH MCS!
The two you mentioned about stoplights are two that drive me completely crazy too.
I'm also over people who "let people in" esp if they're turning Left. I actually did that a few weeks ago against my better judgment and the dude got into an accident. I didn't even realize it until a couple seconds afterwards but I let gave him space to turn left and he cut across our 2 lanes then blasted someone driving in the left turn lane. It wasn't my fault obv, but I'm done letting that happen. It's too dangerous.
People who let in 1 person while 47 people wait behind them while they let that person in also drive me crazy. People actually design where 4 way stops are based on traffic patterns.
My new favorite is going into my daughters school we now have people who will stop and not turn right into the school in order to let people who are turning left into the school turn in, and there are no stop signs/ lights at the entrance. A couple days ago I watched someone just whip around the person "kindly" waiting for hte person to turn left and turn around them and I cheered loudly.
I get that people are trying to be polite but this really causes more cluster than it helps.