I'm not sure why a unique mechanical ability is necessary to define a class. The other nonmagical classes shouldn't have completely exclusive abilities either, perhaps with a few esoteric exceptions.
Because if you don't have a unique feature there is no reason to have X as a seperate class, it should just be folded into some other class. If there were no unique non-magical class features then you don't need rogues, fighters, barbarians, warlords and rangers to be seperate classes. You could just list HD, skills, and combat abilities, pick one from the good list, one from the medium and one from the poor. Plus maybe a special feature like rage or inspiring aura. *click* done, every non-spell casting character can now be described in perhaps 2 pages of the phb.
Barbarian: Good HD, medium skills, poor combat options (finesse is not his strong suit) and rage.
Fighter: Good combat options, medium HD, poor skills.
Rogue: Good skills, medium combat options, poor hd.
Ranger: medium everything.
This is a perfectly valid approach to class construction, but we already know it's not what they are doing.
So. Each class needs to have a defining feature. In 3e the fighter DID have 2 defining class features. One was more feats than anyone else, the other was access to unique feats like weapon specialization (and later mastery.)
And that was enough because feats were new to 3e and presented a brilliant way to specialize and differentiate your two-weapon fighting guy from my archer or his lancer.
In 4e feats were universal, but the power system presented class uniqueness.
We know power style class building is optional for some classes in 5e, so my fighter may not have the cranky badger strike. We know almost nothing about how or if feats will be featured.
So how does the fighter justify his existence as a PC level class when compared to barbarians, rangers and warlords?
He needs something. It could be 3e style feats, but I don't think it will be.
My reading of the current plan is that there is a maneuver system that is somewhere between 4e's page 42, 3e's Bo9s, and a generic power list. Of the iconic 4 classes only fighters can use these maneuvers. The 'sub-class' classes like ranger and paladin also have access to these pools. The fighter is just better at it. And he had damned well better be, or why would you NOT take a Paladin instead?