Infinite already offers a compelling design before you even get to meet Elizabeth. The world is rich enough, and the gameplay exhilarating enough. Infinite does a great job of slowly introducing new mechanics over its intro: from melee combat, to gunplay, to your first encounter with vigors (Infinite's equivalent of tonics). At its core, combat in Infinite should be familiar to players of the original BioShock.
But those sky-lines really change things up. Once they're introduced into combat, you have a whole new way of interacting with and approaching situations. You can still go guns-blazing, but you can also try to get to a higher vantage point by taking advantage of the sky-lines. Also, they are not as intimidating to use as one might expect. You don't have to gauge your jump to connect to one; instead, you simply aim your cursor over where you want to connect, and simply press the Spacebar. Switching tracks and landing on platforms (or foes) is as simple as aiming and clicking once more.
Whilst mid-air, you can fire at other enemies, change your speed, or even go in reverse. While enemies don't chase you on the sky-lines in the early parts of the game, I can already envision later battles ratcheting things up.
The journey to meet Elizabeth is full of mystery
The core tenants of what made BioShock's combat so satisfying return. You can use "Possession" to remotely take over an enemy turret, for example. (You can also upgrade it to take over human enemies, as well!) With Devil's Kiss, you'll be able to set up a fire trap against a teleporting enemy. However, having Elizabeth join your party introduces even more options for you to consider in battle. For example, in one point in the game, I was able to choose between having hooks to grapple onto, or a turret to blast at enemies. I chose the latter, only to realize how valuable getting a better vantage point would have been.
Elizabeth not only provides color commentary for the world Booker is in, but she can be a rather useful ally at times, throwing ammo, health, and money your way whenever the situation calls for it. And don't worry about having to "protect" her; it appears that is not a concern (at least in the beginning portions of the game). Not only does she not get in the way of fights, it doesn't appear any enemies want to directly attack her, either.
It's not often I find myself so captured by a world. However, Irrational has crafted something truly extraordinary with Infinite. Even after two hours with the game, I know I'm barely scratching the surface, and I cannot wait to see more. It's going to take quite a lot to make all these disparate elements come together--but if Irrational succeeds, the payoff will be truly extraordinary.
Sky-lines are surprisingly easy to use