Making an army list in Age of Sigmar is an unusual process for those of us used to the traditional Games Workshop methods. In Warhammer Fantasy you decided on a points value, an army, and then tried to match a unit selection based on force organization rules and a balanced strategy to defeat as many types of opponents as possible. Those opponents would most often be similarly organized. In Age of Sigmar, however, there are only two ways to build for “strength:” by sheer force of numbers, or by maximizing keyword synergies. The sheer-force-of-numbers (a.k.a. pay-to-win) approach will not win you any friends, and may get you banned from games. The keyword synergy approach, though, is as yet an undiscovered country. Everyone is starting to pick up on the idea that you can take certain units to play on their collective strengths. As the community finds its feet in this new game, strong strategies are bound to come out.
For example, the Dark Elf Cold One Knights are somewhat lackluster by themselves. However, in the presence of a Dreadlord on Cold One they (and Cold One Chariots) reroll charge rolls and add 1 to Wound rolls for one of their attacks. It's combinations like this that will define Age of Sigmar army lists. While you can make all kinds of interesting setups, it's this system that's going to draw the tactical minds out of its players. I'm reminded heavily of Magic: the Gathering or Hearthstone here. Both of those games encourage players to build winning strategies on specific synergistic themes that amplify their benefits, while not discouraging taking completely odd selections for flavor or for backup strategies. For my part, I intend on running a lot of Dark Elves (or Exiles, as they're now called) as the core of my first army. There's a lot to love there.