With the caveat that I'm on the anti-warlord committee, but perhaps with the perspective that gives me, the most non-negotiable and thus most contentious characteristics seem to be:
1) The general ability to support/buff your allies, but most specifically by sacrificing one of your Actions in order to grant a bonus Action to somebody else.
2) The general ability to heal, but specifically without using magic*. Some proponents seem to be ok with temporary hit points or even damage mitigation, but that's not consensus. Many insist it must be healing equal to a cleric.
3) The description of the class as an "officer" who "inspires" his allies with "natural leadership" while "commanding" them to do stuff. Few people will come right out and admit this is an intrinsic part of the class, but that fluff is always there.
4) Otherwise it looks like a Fighter.
*(There is some suspicion that the "non-magical healing" stuff is just a proxy fight over whether hit points are meat or not.)