Don't be too reluctant to close questions. Closing is an important mechanism that is very different from deletion - it means that the question has significant issues that must be addressed before someone writes an answer for it. Think of it more like a "stop sign" than killing the question.
Voting to close is much preferred to flagging for deletion (junk and spam posts aside) because that gives all other users an opportunity to participate in the moderation. It also gives the OP a chance to fix their question so that it is acceptable for the community. Your 1/5 close vote lets you express your opinion on whether the question belongs or should go without allowing you to abuse power.
Ideally, the diamond moderators shouldn't be doing the majority of closing, since once a mod decides to close a question, it is closed immediately (no 5 votes to close). However, a lot of questions that in our opinion need to be closed don't get the 5 close votes.
The basic lifetime of a bad question can be sorta summed up with this flowchart. We go through and delete closed questions every so often so they don't build up.
Bad Question
|
Spam/Junk? ----- Yes -----> Delete
|
No
|
Close Question
|
Has question been
improved? ---- Yes --> Re-open
|
No
|
Delete