My pet peeve is great software with incomplete documentation.
All that hard work put into the code, just give us a quick example of how to actually use it! Especially where there is maybe a super superficial example, but missing examples of how to use the optional yet essential and non-straight forward arguments/features.
Oh! I have a much bigger pet peeve for projects which have a fine and moderately-usable examples section, but then decide that's good enough and never document the API because one should just "look at the examples".
All that hard work put into the code, just give us a quick example of how to actually use it! Especially where there is maybe a super superficial example, but missing examples of how to use the optional yet essential and non-straight forward arguments/features.