Inconsiderateness in general. It's a broader term than most specific things, but it sums up my biggest irritation. I.e. people who can't think beyond how their action impacts them to think about how it impacts others.
Some examples -
Somebody doesn't bother to push their shopping cart back to the cart station - Now you've blocked a parking space, made some high school kid have to collect your cart, and increased the potential for auto damage because you couldn't be bothered to walk 15 yards with your cart.
Somebody doesn't use their turn signal - Yes, I know it's in the OP's example, but not using it causes people behind you to not know what you're doing or why you're slowing down...all because you're too lazy to flick a stick.
Somebody parks their car in the fire lane of a store parking lot while their SO runs in to get something - You've created a bottle-neck at the busiest point of a store parking lot so you don't have to be bothered to go park in a space like everyone else.
Smokers who aren't even the least bit considerate about their smoke - Pretty obvious. If you want to smoke, that's your prerogative, but be considerate. If there are people eating on an outdoor patio, don't go stand right up wind from them and smoke. I know there are so many laws about where you can smoke these days...they're there because people couldn't be considerate from the start.
I could go on, but you get the point...Think more broadly than just yourself.