Spoiler: We are humans. We aren't perfect in the slightest bit.
Just because you think your game is amazing and perfect when you play tested it doesn't mean it is. You should get around 3 people minimum with different opinions to playtest your game. Why? Because they aren't YOU and won't go "Omg this is so amazing wow omg i can only give praise!". For those thinking I'm just a big fat meanie head and hate people (latter is somewhat true) please at bare minimum listen to the idea of people trying it out, having them say their GENUINE thoughts and improving your game based upon it. Example: Lets say you make a mod and it's all "finished" right? You have two options:
A. Play it yourself to check for issues.
B. Have other people play it and wait for responses from them to see if there is any issues.
Now you might be thinking yeah option A sounds better because I don't need to wait for separate people to take their time and then respond but here is the thing. YOU MADE THE DAMN THING. Now granted this means you can tell if something is off and wrong but you know WHAT is supposed to happen. Lets say for instance you make a puzzle in your game right? Now imagine if for anyone else who's playing the game the puzzle is really vague from what you are told, but lets say only YOU play tested the game. You would think nothing is wrong and the puzzle isn't vague but when you release it and then everyone starts telling you the said puzzle is vague. Now you can respond to this in many ways but you could of prevented this by well... having other people playtesting it. Now that might be specific so why about we talk about COMBAT. Combat is really hard to balance and get right and if you only playtest it yourself then you'll have a issue where the game can end up busted as balls but you didn't notice or care and it becomes a big issue when others play it. Now what you do with my little rant is up to you. You can take this advice to heart, ignore it, call me a retard, whatever. But if you are going to say "Well I made the mod, I play tested it, so long as I enjoy it I don't care what others think" then i'd say "well why are you releasing it on the internet where everyone is gonna say their opinion (and it's not nice majority of the time)" Other thing. Make sure to take good criticism to heart and if someone goes "its ass" without explanation, don't get riled up about it, just either ignore it or ask them to explain further. Tired now but I hope this got my thing across... maybe... whatever. Again as mentioned before do whatever if you bothered reading this. Agree, disagree, call me a piece of shit who should kill themself, etc etc whatever.
Tl;dr: Have people with different opinions playtest your game to get the most criticism without needing to launch a broken/buggy product out and improve with helpful criticism.
Also we should get cheese rolling contests into the next Olympics.