Part of creating a really compelling board game is including a variety of game mechanics. The classic American board game style of rolling a die and moving along a path is old hat (and was never really that fun in the first place). Great games include so much more than that, like drafting cards, auctioning items, and strategically building your own hand of cards (called Deck Building). Board Game Geek has a pretty great way to browse games by game mechanics if you're interested in a particular mechanic.
Deck Building is a game mechanic that's been growing in popularity ever since the launch of Dominion more than 10 years ago. It involves collecting cards over the course of the game in order to build a reusable hand that is strategically specialized to achieve your unique goals. For instance, let's imagine a game where you play cards to accomplish tasks on your turn. Let's say that you can accomplish different types of tasks with those cards–some are antagonistic, some just earn you points, and some award you bonuses to use later. Imagine that part of your turn allows you to collect more cards to put in your personal deck and when you play a card you don't discard it at all–instead you shuffle it back into your deck. This is the basis of a Deck Building game. You have to choose whether you want to collect more cards to attack your opponents, earn bonuses, or rack up victory points. Depending on the state of the game you may make different choices. Most importantly, you and your opponents will likely make different choices and that's what makes the mechanic interesting. Of course, there are many variations that can be made on this basic game.
What's the point of allowing each opponent to build their own deck anyway? To answer that question we need to talk about Tic Tac Toe. It's a really unfortunate game that is incredibly boring if you are observant. On any given turn there is always a clear best move to make. If you're paying attention, you will always end Tic Tac Toe in a draw. That's super boring. Introducing new mechanics like Deck Building allow players to make different choices and, assuming the game is well-balanced, players will be incentivized to make different choices. As soon as there are multiple legitimate choices to make in a game things get very interesting. Sure, there are many ways to accomplish that, but Deck Building is a great mechanic I've been interested in lately.
One of my favorite games that makes use of this technique is called The Quacks of Quedlinburg. Interestingly enough, in this game each player has a bag of tiles rather than a deck of cards, but the concept is exactly the same. You go about deciding as you play which types of tiles you want to add to your bag. Since all your tiles get put back into your bag at the end of each round, your options change continuously throughout the game. It's a pretty lightweight game that anyone can figure out quickly and I highly recommend it.
Which of your favorite games make use of a Deck Building game mechanic?