Wrote this for people wondering what difficulty to start with if they get the game.
Bioshock Infinite and Difficulty Balance
The difficulty in Infinite pretty much shit. The game gives you the ability to switch between difficulties on the fly (which is really nice), which let me experiment with a few things. My conclusions are that normal mode is mind-numbingly easy, and hard mode is extremely time-consuming.
I have yet to try the ultra-hard difficulty because I didn't know how to unlock it when I started playing.
So in normal mode, headshots with specific weapons are insta-kills which is nice, because the game has a very urgent pace and so those precious few headshots you make while frantically running around become really valuable. Hard mode ditches this for the most part. It seems headshots do more damage, but the general idea is that you'll have to fire a lot more bullets to take anybody down. Takes too long.
In normal mode, turrets are pushovers. You can kind of just stand there and shoot at them, whereas in hard mode they must be feared. They can bring you from max to low hp in a matter of seconds, and force you to time when they'll stop firing so you can find new cover and hopefully get into position to posses your target.
Boss monsters. I don't know what to call these guys without spoiling anything, but there are harder than normal enemies you face every once in a while. In normal they are decently challenging, mostly because you very rarely face them alone. Like the first Bioshock, you fight lots of enemy waves with these big guys included. In hard mode these guy's are bullet sponges. It takes forever to take them down, even when you're firing at their very obvious weak points. Since most of these fights just sort of happen, you can't really plan out your course of attack like you could with big daddies. In normal, I just ran backwards while shooting, and in hard mode I just found the most effective vigor (psychic power) and spammed it.
In normal mode, health and salt is fucking everywhere. Hard mode definitely wins out here. Health, salt, and ammo are rare so when your partner throws you an item it really matters.
Also there are enemies who, after you have meleed them, will immediately fire a rocket/grenade at you from point blank range. This means that melee is noneffective against them, which is fine, but because of the armor it takes years to actually kill them with weapons. I've found in normal mode if you just shoot them in the head over and over their helmet will eventually come off so you can kill them. This is actually uncharacteristically boring enemy design because most of the enemies are actually fun to fight.
So I'm playing on normal for now, simply because hard mode fights take too long. Also don't think that just because the difficulty is poorly balanced that the game is bad. The set pieces are very nice, and I always found ample cover. The tear mechanics are pretty neat (although they're more about discovery than actually making any decisions), and of course the powers are rad.