1. Very poor: "Thanks" "NP" "+1"
2. Poor: Less than 5 words, Posts without any meaning like "I like your idea and feel everyone should like it", "Posts containing too many emoticons" "Repeating the above posts" etc.
3. Average: Posts that are grammatically poor but the content is helpful to the users, engaging in a conversation without going off topic, giving references, posting meaningful content.
4. Good: Posts that are really helpful and have useful information (may it be technical responses regarding bitcoins, mining, wallet recovery and so on).
5. Excellent: Posts that are made by extremely knowledgeable people who give examples, tips, tricks, tutorials and explain their answers with examples. Their responses may be extremely long but the content is useful to one and all.
Obvious trolls and spammers do not fall in any category.
I agree with 1, 2, 4, and 5. I would qualify 3 to compensate for posters that don't have English as their first language. For example, should we downgrade you post because you say "less than 5 words" rather than " fewer than 5 words".
Also, I think that people asking questions can be valuable members, even though their level of knowledge is low.