So many posts get deleted for "simply" "not being good enough" or "long enough"... absolutely pathetic
You can replace 'not being good enough' with unconstructive or low value. The forum doesn't set high standards for what a post should be, especially in some of the boards. There could be a difference of opinion between what the mod and user considers low quality, but, if it's off topic, or contributes little to the thread then it would likely get deleted.
Some boards are moderated stringently, but there's lots of freedom on the forum.
On the aspect of post length; I do not think a mod would delete an on topic and quality reply cause it's not long enough. Number of characters does but really matter.
If you just add a 'thank you' or 'good job' reply to a thread, it would get deleted as it specifically breaks forum rules.