The first time someone plays your game, their enjoyment is crucial.
Each copy of your game will be played many times by its owner, but by many other players, once. This is because they're playing it at a games club, or after a dinner. That impression really counts.
People will play your game more than once, but only if they enjoy it the first time. By the end of their first game, they must fully understand the game. Preferably, they understand it very early on, and spend their time strategising, not simply trying to understand the rules.
Personally, I have no time for games that aren't good on the first play. Maybe they're great after ten plays. I'm not going to invest the time to find out.
Many fans of Radlands have told me that they've had numerous occasions whereupon they've played the game with someone once, and that person bought the game immediately. I was told that one player even stopped playing mid-game, and started using his phone. When his opponent asked him what he was doing, he said he was buying the game online.
As much as possible, you need to build a ramp for new players, so they can ease into your game.
Include a "beginner setup", which specifies the setup of the game, for the players' first game. This setup should not only the simplest game elements, but also the ones that have the most straightforward effects and benefits.
This lets you keep players away from complex or unusual game elements designed for experienced players.
There is no reason not to do this.
In Catan, the players create a random board out of hexagonal map pieces. Then, they take turns placing two settlements on the board. The beginner setup instead has a picture of the setup, and says "set the board up like this, and put the players' settlements here."
In Radlands, there are dozens of different location cards. Each player is dealt six, and chooses three to keep.
For players who haven't played the game before, the rules instruct them to instead use three specific location cards each. This eliminates the pre-game location choice, which is meaningless to them, and it gives
each player three simple locations, to ease them into the game. I also biased this set of locations towards aggression and normality, so that the players' first game would more likely be a quick, action-packed brawl.
In a complicated game, it can even be worth marking some of the cards in the deck, and giving each player a specific hand of cards to begin the game with.
If your game is very complicated, you should go even further, and create a "family version", where you remove some systems from the game entirely, for the players' first game.
For all but the simplest games, I always use reminder cards.
A reminder card is a card of very brief rules. It's typically used to display data, not large blocks of rules text.
I use reminder cards to reduce the complexity of my rules, not just to provide information in a handy format for the players.
Allow me to explain.
In my gangster game, there are four unique custom dice. During development, I replaced all the boring "do 2 damage" and "gain 3 health" type of die faces. The game now has fifteen different die faces. I made sure each die face was extra-simple, but fifteen is still a very large number. I put all of these on a handy reminder sheet. The rules for the game are barely a single page, and they just say "start playing now. Refer to the reminder sheet to see what the die rolls mean." Players can start playing straight away, and during their first game, they really do learn all the die faces long before the first game ends.