A: Such a short question for such a complicated topic!
Thank goodness so many other people have written on this, as it can be quite hard to explain, and there’s just so much that can be said:

• Let’s start off with some game design tips from Andy Looney himself, since here you are at Looney Labs asking us this question. And here’s Andrew Looney’s Eleven Principles of Game Design. And check out his groovy How I Design A Game flowchart.

And now some of the copious excellent resources available to game designers these days:

• Vast resources from Stonemaier Games on How to Design a Tabletop Game. (Looks like they’ve gotten a lot of questions on this topic themselves!)

• James Mathe’s excellent collection of blogs is curated into different categories, one of which is Game Design, so check it out!

• There’s even an entire online course, called Game Design Concepts. Though it was originally taught in 2009, a lot of the information is still quite valuable.

• If you want something a little more recent, the book Game Design Workshop just came out in its fourth edition as of 2018.

• Here’s a page with reviews of 11 books about game design.

Finally, some people ask: “What makes a game FUN?” If we knew the magic formula that made everyone like a game, we’d be a lot richer. That’s why you playtest your game – with your target audience. Is it for kids? Families? Role-Players? Strategy board gamers? Partygoers? Collectible Card gamers? There are many different types and aspects of games. Some people like some things and hate others, so there’s no way to please everyone. If you’re trying to get somewhere, it helps to know where you’re trying to go.