At the end of every game design, there should be an extended period of "polishing". This is where you analyse and fine-tune the small-scale components of your game.
Pore over every number, every card, and every other little thing in the game.
Every card in your game should have had hours of thought put into it, and it should be the card that survived when you culled so many others.
Your game should not be polished until it's fairly round and smooth. It should be polished to an exact sphere, such that you can't possibly find any fault with it. Fix or remove everything that's kludgy, inconsistent, a bit weird, counterintuitive to players, complex, hard to explain, or not super-enjoyable.
Your job is not to find excuses. It's your job to get rid of these things, and find simpler, nicer ways of doing them.
I want the game to be a breeze to play, and be childishly simple. I want everything to be exciting, bursting with theme, strategically interesting, and fair.
Keep working on it, until you give me what I want. As a player of your game, I have no time for your excuses. I want all good stuff, and no bad stuff. I want everything, and I don't care why you've failed to give it to me.
Polishing includes large-scale issues, like appropriate game duration, catch-up mechanics, and dealing with freak situations.
This polishing phase adds a good chunk of quality to your game. Don't be afraid to go back and make larger changes, if your polishing uncovers larger flaws, or you come up with new ideas. My polishing goes backwards and forwards many times before concluding.
With a properly-polished game, I find that playtesters are apologetic that they can't provide any negative feedback at all.
When one of my games is complete, everyone likes it, and some people love it.
Why do I put such finely-detailed work into my games? Couldn't I make twice as many games, instead of spending so much time adding a small amount of quality to my existing games?
Yes, I could.
However, the success of a game does not scale linearly with the quality of the game.
When people buy games, they do not buy randomly. They buy the best games. This means that a small number of excellent games account for most of the games sold. As your game approaches the quality of that elite group, its success skyrockets.
A game with a "quality" of six out of ten might sell 2,000 copies. At seven out of ten, in might sell 20,000 copies. An eight-out-of-ten game might sell 200,000 copies. You get the idea.
This means that every tiny improvement you make to your already-good game will increase its number of copies sold drastically. Making your 7/10 game into a 7.2/10 game might make it sell twice as many copies.
I'm not trying to make lots of reasonable games. I'm trying to make one game that's a huge hit. Doing this is worth my time, and it's worth yours too.
As game quality increases, success increases exponentially.
A game is not done until you've fixed every single problem — even the ones that you thought were just inherent in your choice of game style. The game should be relatively simple, and every inch of it should be bursting with interesting and relatable things.
At the end of development, you're not fixing problems — you're making things better. You're just going over absolutely everything, until it's perfect.
At the end of Radlands' design, I was very happy with everything in the game. New and experienced players all liked the game, and struggled to find anything negative to say about it. That was good evidence that the game really was done.
There will come a time when you play the game several times, and not want to change anything. That's when you know the game is done.
...and that's when your game isn't done at all.
At the end of design comes what I call the "resting" phase. This is where I'm not setting out to do anything, and there are no problems that need to be fixed. I just play the game for a few months. During this time, I'll see things I could improve. I don't know what these things are in advance, and they're not obvious problems, but the resting phase allows me to see them. Just a few card tweaks here and there are enough to make this phase worthwhile.
The development of both my published games ended with a very frustrating phase. The game would just keep getting better, and was never done. I just let this process go on for as long as it wanted, and eventually, it ended.