I apologize in advance for not having definite criteria/thresholds if that's what you're looking for, but do note that I usually support RfAs, especially if there's no major reason to oppose. There are a few maxims I have, which I list below:

• If NOBIGDEAL is the only reason for me to support a candidate, I will oppose that candidate. That does not mean that I necessarily don't believe it, it's just that I don't feel it should be a " Get the mop free card". Experience in adminny areas such as NPP, UAA, etc., counts as a "reason" for these intents and purposes, so this is hardly ever an issue and I apologize if I come across as menacing.
• Don't be an asshole or a smartass. Civility is key to being an admin; while I'm not going to grill you on this, coming across as insolent, cavalier, and/or rude during an RfA will make it much harder than it needs to be.
• I rarely !vote neutral on an RfA. If I can't support, I'll probably oppose or simply abstain, or go on General comments.
• Edit summary usage is key. The higher the better; if I don't see at least 90% usage or several month's worth of usage being above 90%, I will ask you a question on it, see below.
• Content creation isn't vital, but it helps. As someone with two Four Awards under my belt, I respect candidates with experience in the various Featured Content processes, and I will oppose if a frighteningly negligible proportion of your edits are in the mainspace. Having said all that, admins are first and foremost backstage workers, so I consider experience in those fields more important than content work.

The questions I will ask

The elementary school question (retired)

This is a question I used to ask on every RfA that I had the opportunity to do so. I got this from Kudpung's list and reworded it to be less devilish. It goes something like this:

An editor creates an article on an elementary school that entirely comprises material copied and pasted from that school's website. What criterion for speedy deletion applies, and in particular which criterion/a do(es) not apply?

An acceptable answer is roughly as follows: Copying and pasting material from other websites to Wikipedia is a big no-no, so criterion G12 applies (unless the school's website has a compatible license, as some of the more astute candidates point out); on the other hand, A7 explicitly exempts educational institutes so does not apply in this case. I'll be more or less strict with my interpretation of your answer depending on how much you plan to deal with CSD. This answer by Newslinger (supplemented up to here) is by far the most in-depth answer to this question I've ever received; other candidates certainly needn't go to that level, though it doesn't hurt.

(UPDATE: I have retired this question as of February 2020 as it had worn out its welcome and isn't as clever or useful as I used to think it was. I reserve the right to ask it again, but I'm almost certainly not going to.)

Edit summary usage

If your edit summary usage is not greater than 90% and there hasn't been a trend of >90% edit summary usage for the past couple of months, I will ask you to commit to using edit summaries more often in the future. The only acceptable answer to this question is in the affirmative; communication is vital for adminship and that includes edit summaries. Bonus points are gained if you habitually make dummy edits to fully explain yourself with the edit summary, although you'll likely not be asked this question if you do.

Others

I will hold you to your answer to Question 1 to determine what other question(s) I will ask, if any. So far I've asked the following questions during RfA:

• Is office.bomis.com an appropriate username?
  • The general answer is no, as it is a relic of bygone software and doesn't make a good username for various reasons.

The questions I would ask if they weren't so difficult

I have thought at times about asking RfA candidates questions about rather niche areas of policy, but that would be too mean-spirited and contrary to the purpose of RfA and indeed Wikipedia. I list some of these questions below as a sort of study guide for admin hopefuls, and I leave the answers as an exercise to the reader.

• An IP address in the 156.33.0.0/16 range is engaged in a major spate of vandalism. What special procedures need to be done about dealing with it, if any?
• What is the copyright status of this edit?

Candidate usernames

I do not oppose a candidate based on his or her username, and I believe that it is the silliest reason to oppose a candidate. Although there are always exceptions, in general if a candidate's username is genuinely problematic then it would either have been addressed in WP:UAA or WP:RFC/N, either way before the candidate got to the RfA process. By that same token I don't support someone based solely on username either (although no one does, to the best of my knowledge).