Most of these are from a wikihow page, with some modifications and clarifications- Just thought I would point that out there before somebody else noticed and started screaming "PHONY!"

"Criticism is to improve oneself and ones life, but not to insult or disgrace someone."
We use criticism as a system of finding and correcting possible faults to achieve a better end result.
If done properly, criticism is a good way to help us all improve our work by getting bits of feedback from other people, and their thoughts to help better the product.

If criticism is not done properly, for example: by one who is not a mature, diplomatic person, it may backfire. Backfired criticism leads to what may appear to be personal attacks or vastly opinionated arguments.

1: Make sure you have a good understanding of what your criticizing before you do such. Put yourself in their shoes, see things how they might see them. Think how you would like to be treated if this was your own content and other people were criticizing it.

2: Criticism should address any unclear areas and end with ways to fix them or improve them. Saying "The stairs suck," Is not criticism, it's an opinion. Instead say, "You get hung up on the stairs, this could be fixed by clipping each step."

3: Address the good things along side the bad. Do your best to refrain from making the person who's work you are criticizing feel like crap. For an example "The stairs need a lot of work, though the architecture supporting the ceiling is quite well done." This complements the work but also shows where a problem lies.

4: Do not criticize the Person for their work, criticize the work.
One great mistake is to point out the errors of the person and not their work, refrain from insulting people or putting them down because you dislike their content.

5: Don't use the word "You," as it can make what your saying seem like a personal attack, even if your not intending it to. Odds are the person will reply defensively. This is a common thing that is prone to start arguments, like that of which occurred in a node here not long ago.
As an example "You made a large error on the wall that causes players to get stuck," This could be changed to "There is an error in the one wall that causes players to get stuck." Some people will put emphasis on the "You" when they read it which sometimes make it seem more like an accusation.

6: Check over what you've said before you post it. Improper wording can lead to people interpreting what your saying as something different entirely. We don't have tone of voice in text, and if you word something improperly then someone may see it as an insult.
Imagine somebody saying "Great work!" First imagine it in a rewarding, kind tone, then in a sarcastic tone. The difference is made by the text leading up to the words, where as in real conversation the difference is shown in the way we say the words.

7: I can't put enough emphasis on this, but check your spelling and grammar! This goes hand in hand with the previous point, and a criticism that nobody can understand is of little use to anybody. If your horrible at english, or just typing in general, thats alright as long as you try your best. If all else fails then try an online app: for example www.SpellChecker.net
Once your done, read it over a time or two.

This post was read over three times and revised twice.