I absolutely hate it when people drop my name and just 'hi' me, or first name me. Literally drives me insane and makes me question what I have done to piss them off.
If its formal and someone I don't know, I'm not too arsed. But people I work with on a day to day basis, it just passes me off. Doesn't take much to be personal and polite. There is one guy in particular and we used to be good friends but have drifted a bit with working from home, he is senior to me and I need to have email contact at least once a day really. I am always polite and 'hi name' him, sometimes 'hope you are well' I rarely get anything polite back. Usually a 'hi' or even worse no hi or hello, just straight into a response.
It annoys me so much!!!!
Also, when you email someone something urgent and they fail to respond. But an hour later a really mundane pointless email they don't need to respond to from someone else lands with them and also to you, and they respond within a couple of minutes. That also makes me feel like I've pissed them off.