Bioshock Infinite shares many of its gameplay mechanics with its predecessors, albeit they are moderately retooled and renamed. Weapons are still upgradable, but you're limited to two. Heavy Hitters fill the role of Big Daddies, injectable Plasmids are replaced by consumable Vigors, and Gene Tonics are replaced by Gear. Here is where I depart.
Gene Tonics were the passive portion of Bioshock's character customization, and fell into three categories, with a cap of six tonics equipped per category. Combat tonics affect the ability to fight: they can shield the player from harm or make their weapons more deadly. Engineering tonics change the hacking minigame and reduce prices at vending machines. Physical tonics affect the character's fitness: allowing him to heal more easily, move faster, or hide from enemies. Found around Rapture or developed by researching splicers, Gene Tonics occupy the fiction in much the same way that plasmids do; both can be bought at Gatherer's Gardens, they look similar, and both are clearly meant to be products that are marketed to Rapture's citizens. Splicers appear to prefer them to plasmids, in fact, since many splicers have marked physical deformities that may result from abuse of tonics like Sports Boost, Armored Shell, or even Static Shock, but no splicers ever visibly use a plasmid that is available to the player.
The major problem, however, is that the gear is completely divorced from the story. There is no explanation about why a particular piece of gear causes its effect, and although they're implied to be articles of equipment, there are no models for any of the gear. Oftentimes the relationship between the item and its slot are completely nonsensical: you get a pair of boots called "Tunnel Vision"... which reward the player for shooting down the sights. Gearboxes are also randomized, so opportunities to imply a story by leaving a particular piece of gear in a specific location are gone. In fact, the only time the game ever appears to acknowledge gear as anything other than a cursory expression of game mechanics is a vandalized tailor shop with a box of gear in front of it, and even that's a pretty threadbare thing.
Why does this frustrate me so much, you might ask, if you haven't already guessed that I'm a crazy person and gone on to more fulfilling ways to spend your day? Obviously, because it's a passel of wasted opportunity. Gear is the final version of what were billed through the game's development as "nostrums". The term describes patent medicine and snake oils, and in that form would have been a perfect companion to the Vigors. It would have grounded them in a mutual universe.
So I have to ask: why gear? Why was a perfectly functional piece of fiction abandoned for an inferior mechanic? My guess is that they thought it wouldn't matter, and of course, it really, really doesn't, does it?