In case it's not obvious: discouraged customers are less likely to buy your product.
An example of this is the pre-order discount. This is a common tactic in the hobby games industry, particularly among wargame manufacturers where it's become a nearly universal practice. It obviously works for them on some level, but it does tend to discourage new customers in a market that already struggles a great deal to attract new customers. Sometimes the pre-order discount is so large that you have to wonder whether the final MSRP is being overly inflated to further encourage pre-ordering.
Pricing isn't the only way to discourage customers. Another way is the pre-order exclusive. If the only way for a customer to get a certain substantial thing is to pre-order, and they missed the pre-order, then they're even more likely to become discouraged than they would from missing a discount. By substantial I mean something related to game-play. An adventure, or scenario, or expansion that's exclusive to pre-orders and otherwise unavailable.
In the past I've even seen companies that only offer a legitimate PDF of the product to those who pre-order. This goes beyond discouraging sales to actively preventing them!
Substantial exclusives in general are a bad idea. Whether it's a pre-order exclusive, a con-exclusive, or a direct order exclusive, when customers find out that they've missed out on something, they become discouraged. That means while they may have already bought your core game, now they're less likely to buy anything else related to it.
A combination of the two is the pre-order bundle. Everything in the bundle is available after the pre-order, but at a higher combined price. This is most commonly found in situations where the PDF is offered free with a pre-order, but has to be paid for separately if you don't pre-order.
So, what's a company to do if they want to encourage pre-orders, but don't want to discourage late adopters? Offer bonuses that aren't involved with game play: signed copies, t-shirts, posters, anything that's "cool" but isn't actually used in the play of the game.
Many Kickstarter projects are using this kind of thing effectively. In many cases people are actually paying extra to be an early adopter (in some cases a LOT extra) if it gets them something cool, even though it's not related to the game play. Something as simple as getting your name in the product as a supporter can help encourage people to put money up front.
It's possible to get creative about offering incentives while avoiding dis-incentives.