What sort of stuff do you do to modify your forum?
On MyBB, I recommend Google SEO, reCAPTCHA, Stop Forum Spam, the Trash Can Forum (don't know why this is not a default feature), Quick Theme, and Advanced Quick Reply Forum. Other than that, I think it's all personal favourites.
I like Google SEO and AutoMedia for MyBB.
I forgot to list Image Resize. That should've been default as well.
The cash system because you can give it to someone that has helped you, buy things with it, play around with the lottery feature etc.
I really like the points plugin but lot of forums has it now and there is nothing new about it.