Are there good enough incentives for a fighter to wear light armor, though?
Well, let's see.
Unless you want a double-digit damage die (say
that five times fast!), it doesn't matter whether you specialize in Dexterity or Strength; that is, at least one d8 weapon uses either your Str or your Dex for hit and damage. This is sort of a mixed thing: on the one hand, there's a "disincentive" in that you can't get the
best damage dice with Dexterity, but on the other hand, Dex-based fighting has never been easier nor has it shared as much of the "damage space" as it does in 5e. On the flipside, you cannot carry those weapons
and use a shield--but two-weapon fighting (which massively favors Dex-based) generally falls behind since 5e puts such emphasis on number of attacks as an advancement mechanic. I'll call this a slight win for Dex, simply because they've removed the hassle of picking up "Finesse Fighting" or the like.
Dexterity powers initiative, AC in light armor (max +2 in medium armor), the Dexterity save, and ranged attack hit and damage values, IIRC. Initiative has always been a great thing to improve. One stat that governs both melee and ranged attacks? That, too, sounds like a pretty strong incentive. On top of that, it affects one of the most common saves (what would in 3e be called a Reflex save, and in 4e the Reflex defense), and improves your general defenses as well. Dexterity checks, particularly for legerdemain, acrobatics, and stealth, bring lots of utility. Further, light armor (plus max Dex, of course) keeps pace with heavy armor--so there's neither a loss nor a gain in AC, relatively speaking, yet that Dex bonus (AFAIK) cannot be lost while armor can.
Strength improves melee accuracy and damage, carrying capacity, and "athletic" skills like running, swimming, marching, etc. If you have the proficiencies, Strength lets you wear the heaviest armor and wield the highest-die weapons (d10s and d12s), though (as noted) this denies you a shield. Strength saves are rarer (I don't know the official numbers for the core books, but in the playtest there were only a handful--half a dozen at most), but from what I've heard they
can be somewhat nasty, much like Int saves. Strength does nothing to directly improve defenses otherwise.
So we have, from Dex: Speed, melee accuracy and damage, ranged accuracy and damage, common magical defense, physical defense, stealth, ancillary skills. On the minus side, you can't get the absolute best damage dice, and if you outright
dump Strength you may end up with a terrible carrying capacity.
From Str: Melee accuracy and damage, carrying capacity, higher possible damage dice, highest AC from items, "athletic" skills/checks, rare (but dangerous?) magical defense. On the plus side, high Str = carrying capacity = alternate weapon sets (there
are Str-based thrown weapons, IIRC). So Str isn't completely inflexible, just less so than Dexterity.
Admittedly, I may be missing things--I'm tired, and I already believe that Dexterity is very much the "god stat" of melee characters in 5e. But as it stands, I'm pretty sure Dexterity is outright superior, numerically, unless one goes in with a very particular build in mind (for example, polearms don't really work with a Dex build IIRC, though I've heard they require 1-2 feats to really shine.)
Edit:
And all of the above is with (essentially) zero knowledge of the feats available in 5e. Throw in the aforementioned stuff like Sharpshooter and Crossbow Expert and things are even nicer for the Dex Fighter. I'd also say that any subclass of Fighter can do well with it--EKs get spells to enhance mobility and defenses; BMs get maneuvers; Champions get the second fighting style, and thus can have both the Archery bonus
and their pick of the remaining set (probably Dueling or Defense).