Co-Op Board Games - The Good & The Bad

I am looking to pull some information together and would like to see a wider prospective than just my gaming experience. Would like anyone to participate with answering the following question. Please be as respectful as possible to other. This is their opinion on the question and my not match your views. I am not looking for the name of the game to be called out, unless you feel it's a must. 

1. Best non-combat game mechanic. 

2. Worst non-combat game mechanic. 

3. Best Co-op combat mechanic. 

4. Worst Co-op combat mechanic. 

5. Best way a game has spawned the enemy or objective. 

6. Worst way a game has spawned the enemy or objective. 

7. Other good thing a co-op game has done. 

8. "I wish they hadn't of done that"

All of the previous questions pertain to Co-op board game, but could include a game the also allows solo play. (example D&D Castle Ravenloft allows 1-5 players is a co-op game, but still allows solo play.)

Thanks to anyone who answers. Looking forward to reading what people have to say.