It would be useful to have some examples of questions that should and shouldn't be closed for specific reasons, with detailed explainations why.
It could reduce the number of invalid flags and make life a bit easier for users new to reviewing. Getting used to the new close reasons could be smoother too.
It's not always obvious if a close reason is applicable to a question or not, descriptions are sometimes too short to resolve doubts. Some questions are balancing on the edge of being valid for SU. Sometimes I can see questions being almost closed (4 close votes) despite being perfectly fine in my opinion. I guess sometimes I'm the one who casts the invalid close vote, too.
I know there are many questions regarding closing here on MSU, but that's the thing - there's too many of them (see close-reasons tag). Having all useful tips gathered in one place and backed with examples would be really nice. It could be even better if such guide is linked in the closing pop-up.