Holding open a door, allowing someone into traffic, waiting for a pedestrian to cross the road, handing someone a tissue if about to sneeze, letting someone pass through doors or stairs before me etc.
These are all things that I have made the choice to do. No one forced me. I do them because I want to. So I really don't care if for any reason someone doesn't thank me for it. I didn't do it for the thank you, I don't think they people are the ultimate of rude for not thanking me. I know there life in the tiny second was easier because of me, why would I go and get all bothered by them not showing it?
And before anyone makes an assumption, I always thank, because its just such a reflex now but i think people are prtty for needing it. And yes, I do think it is rude not thank for bigger things or for something you have actually asked someone to do for you.