What makes a great leader in todays military?

The best leaders I've ever worked for all genuinely cared about their subordinates. They would take time to mentor and coach their subordinates to be successful professionally and personally. It can't be faked and there is no substitute to looking out for the welfare of those placed under your charge.

No single thing, but a combination of lots of little things.

Leadership, is like art. All its components are Science or Math based. You can read them in a book. However, their APPLICATION is what makes it good or bad art (leadership).

We teach fundamentals & technique (science). The person has to use intuition to apply them (art).

You did touch on an important point, "perception is reality". A good leader/mentor is more than the words that come from their lips, it's about the subtle things they display when no one is looking.
I recall, way back in the late 80's, when I was an E-3. On Fridays, we had a general clean up of the warehouse facilities and once everything was satisfactory we usually had a burger burn and some downtime. Well, my coworkers and I half-assed the cleanup so we could get to drinking a couple beers and eating sooner.
Our NCOIC had a walk around and could tell that we slacked. Instead of chewing our asses, as he had every right to, he grabbed a broom and sweeping compound then began to do the work we had shirked. I immediately felt a flush of guilt and attempted to "make it right" and asked for the broom. He said no, I've got this, enjoy yourselves. To say I was embarrassed would be an understatement.
That was a true leadership/mentoring moment to me that I carried throughout the rest of my years of service. Not to mention, I recall finding those burgers exceptionally hard to swallow.