None, it's amazing.
The barrel is super accurate
The trigger works great
The stock board is good
Barrel, trigger, board, weight, internals, easy to maintain.
All I have to say is wow, this is an amazing paintball marker for such a low low price. I know lots of people who own this marker because of it's high end quality for only 325 dollars!
I used to own a 16 inch stifi barrel for it, but when comparing the stifi and stock barrel, I found out that they had the same spread. At very long distances you may need a better barrel, but for speedball the stock barrel is the way to go.
I really liked the trigger on the mini. It has a bit of bounce but is still really sensitive. It works perfect for me. You can also adjust the trigger if you want it to be more sensitive.
The mini is also very small and very light. The mini's size is perfect for bunkering people, very easy to wrap around corners to shoot people.
I'd give the mini an 11 out of ten for being such and exceptional paintball marker.