Abstract things that annoy me
The sentiment ‘it's not much to ask’, presented as the sole justification of why people should do something you want. When everybody has a ‘not much to ask’ request and they're all different (or, occasionally, when the same person thinks of a different one every week for a year), they add up until collectively they are a lot to ask - so some of them have to go unfulfilled, despite each of them individually being so small that the asker couldn't imagine how anyone might have a good reason not to do it.
Don't just point out that the cost is low and leave it at that. Show why the benefit (whether to you, to whoever you're asking for it, or to somebody else) outweighs it!