I am sympathetic to a lot of bad gym etiquette cause we all were new at one time. if they get in my way I politely tell them about it. example i had some kids take my weights off the ground I was using for leverage on my bench press. (they obviously did not understand why the weight was on the ground, unless a person competitively lifted at one time they may not know that is totally legal to do for us guys with shorter legs) I told them I was using that weight with a smile and they gave it back to me.
another time I had X amount of weight on and the personal trainer who was training a female took my 2.5 pounds off my bar on one side.. I had no idea they had taken it cause I had a 45, 25, 10, 5 etc on there.. luckily a buddy of mine noticed it.. could of been a potentially dangerous situation having more weight on one side of the bar cause of that PT's mistake (i'm assuming they thought no one was using it and looking back i should of told them something) i said nothing to them and just grabbed another 2.5 pound weight.. not a big deal and not worth losing focus over. so it all just depends on the particular situation.
I like to stay focused in the gym and as long as they don't get in my way I'm good.. luckily my gym isn't crowded so things like doing curls on the squat rack is not an issue but if i worked out at a crowded gym I would definately have a problem