Wikipedia:Passive aggression

Passive aggression is behavior "characterized by the expression of negative feelings, resentment, and aggression in an unassertive passive way (as through procrastination and stubbornness)". The basic meaning of this is degrading and insulting others in a way that makes it sound neutral and not harmful. Being passive aggressive to others on Wikipedia makes you look unfriendly, and new editors will be less likely to ask you for help. However, you still need to say what needs to be said in tough situations.

What to do

 * Tell them you think they are wrong
 * Tell them why you think they are wrong
 * Tell them how they could fix the problem
 * Do so in a neutral, friendly way
 * If it's not important to you or you don't feel like it, you don't need to add anything

What not to do

 * Insult them openly
 * Insult them passive-aggressively yourself
 * Tell them they are right if they are wrong
 * Be overly kind (it might look like you support them even if you don't)

Examples
Say an inexperienced editor (Editor A) who has only been on Wikipedia for several weeks and has only made several hundred edits requests adminship. Two experienced editors, (Editors B and C) have both been editing Wikipedia for a few years, and have made thousands of edits. They write on Editor A's requests for adminship page:

Editor B: "You have obviously little idea on being an administrator, so I'll get this over with and vote you down. You have made virtually no edits, and by no means should become an administrator within the next several years. Is this really who's wanting to become an administrator now?"

Editor C: "I believe that you cannot become an administrator as of now, due to lack of experience and edits. However, once you surpass the number of edits and the amount of time spent on Wikipedia that most administrators have, you should try again. I look forward to working with you on Wikipedia."

Notice how Editor B tries to inflict as much damage to Editor A without openly attacking. A message like this might scare Editor A into retiring from Wikipedia prematurely. However, Editor C also opposes Editor A's request, but in a neutral and helpful way. This message makes Editor A feel like an equal, but doesn't promise them anything impossible that would make them feel better. This example shows that it is easy and possible to tell another editor something they don't want to hear without being mean. Of course, this rule isn't just related to requests for adminship.

Use common sense
Of course, if an editor is being especially mean to you or someone who seems unable to defend themselves, you don't have to be neutral and friendly. However, being passive aggressive to them makes you look just as bad. Tell them to stop, and take action any way you deem necessary.