I propose the following groups by which people might identify themselves when they post a song:

1) NO JUDGMENT REQUESTED
this person just wants to post and will be offended if we offer suggestions. But, in all fairness, anybody who posts this way shouldn't get to hear praise either.
"No comments is no comments"

2) EXPERIMENTAL
this person isn't inrterested in following the "rules" of recording, they just want to try things that sound interesting. Appropriate commentary for this person would be to say whether it hit the mark of being interesting or not, and maybe mentioning thoughts that might have made it MORE interesting

3) NOVICE
This person is at stage 1 in the learning curve. They mostly need to hear what they did right. Too much criticism too soon will make people stop submitting

4) PERFORMER
This person has experience playing live, but that doesn't necessarily translate well into the recording world. Tips here might focus on differentiating between what works on stage as opposed to what works in a studio. This person is actively committed to learning how to write and record songs. A person at this level may be on drugs and want to kill you if you criticize him. Hah ha, just kidding. Well, maybe not in all cases. Test the waters before committing too much criticism.

5) APPRENTICE
Anyone at this level should be hungry for advice, good and bad

6) JOURNEYMAN
This person has been around and had his share of successes, and can probably do a lot of things right. I'm thinking this person might be more resistant to criticism than some, because in his mind, he already knows how to do this. Gentle suggestions rather than outright criticism would probably work best with this person

7) EXPERT
This person is skilled at writing, arranging, performing, mixing and mastering, but is not a paid professional. AT this level, some nit-picking should be acceptable, because there shouldn't be any glaring problems anymore

8) PROFESSIONAL
this person may offer paid mastering services to the public or have a commercial studio. Most of us will probably not hear any problems with this person's submissions, and even if we did, the person would probably plead artistic license. But a person at this level should be proficient in all aspects and therefore subject to any and all criticism