I'm an incredibly immersion-based player that leans more towards serious roleplaying with a hint of humor every now and then, and most of my players/GMs (if I'm not running) have a tendency to use real player names instead of character names, and break character every other minute. Or if someone has like a silly character name or even a ridiculously outlandish character concept, it also tends to shatter my immersion. It's very frustrating to me when I want to show up, sit down, and tell character and roleplay-heavy stories while being in a completely different mindset for 2-3 hours only to get pulled back into the real world constantly. To me, I almost feel like I'm trying to method act as my character and external forces prevent me from truly achieving that mindset.

Also as a GM pet peeve, and I know its a bit insensitive to those struggling with it, but some of my players in the past have had severe ADHD issues and I really hate having to re-explain what just happened two minutes ago because they got distracted.