I thought this would be both healthy for the community to discuss and fun to see peoples opionions.

For me, supportive posts have elements of the following:

-Some level of empathy from the poster. This can be a hug, an "I can relate", an "im sorry that you experienced that"
-A similar story/situation which shows I am not alone in what I am posting about
-Advice that is even toned and offered in a way that is not overbearing or harshly critical. An example would be saying "It might be good if you tried *THIS*" Instead of "Oh, you are a bad person for doing *this* and you HAVE to do *THIS*
-Resources related to what I am posting about

This is certainly by no means all of the things, just some ideas.