Failing to draw a distinction between any worthwhile things. There's a particular way in which a concept – a noun or adjective, or a philosophical term of art – can fail to be practically useful: by being defined in such a way that nothing, or nothing interesting, satisfies its definition. Or else nothing (interesting) doesn't satisfy its definition. As soon as you notice you're talking about a concept which encompasses either everything or nothing, you're usually wasting your time (except in the rare cases where you really intended to be talking about either everything or nothing), and should instead be looking for some alternative concept (or a less absolutist interpretation of the same concept) which manages to draw a dividing line such that at least one interesting thing falls on each side of it.

I want a word for that particular form of uselessness, so that I can much more economically point out when somebody (certainly including me) has perpetrated it, and it doesn't take me a whole paragraph just to explain why I'm giving up and trying a different approach.

Moral versus tactical ‘should’. It keeps striking me as an unfortunate property of English that the word ‘should’, and many of its synonyms and related words, are sometimes used to indicate moral obligation and sometimes used to indicate the tactically (or strategically) optimal course of action. Usually it's obvious from context which sense is meant in any given case, but not always, and I've seen just a couple too many arguments flare up from somebody misconstruing a ‘should’ as moral when it was intended tactically, or (more rarely) vice versa. It's a pain to keep tacking on disambiguating parentheses such as ‘(I'm speaking in the tactical sense here)’, so I want two clearly different words that can be used in place of the ambiguous ‘should’, at least in sensitive circumstances and perhaps more widely too.