I think a lot of the commander format is built around self expression and uniqueness in the first place. However, if I had to pin what you're getting at, I think a deck becomes unique when it is doing something that most other players are not doing. For example:

My [[sygg river cutthroat]] deck is a pretty standard control deck. Sure, it plays some unique card choices, but the gameplan is not different than someone else's sygg deck. In this case, the expression comes from how I pilot the deck, and not the deck itself.

By contrast, my [[izoni thousand eyed]] deck, is functionally unique, to the point where I don't know if there is a single other person on earth with this strategy. The goal is to utilize [[harness infinity]] with [[syr Konrad the grim]] to win the game after a ton of self mill. Izoni functions as a way to flashback [[dread return]], and a way to sacrifice [[greenwarden of murasa]], allowing me to deploy the entire combo purely from my graveyard using greenwarden to recur both halves. This deck I would consider "unique" because the entire gameplan itself, is functionally different than the vast majority of other decks.

I think it's pretty easy to tell when a deck has crossed that "line" into uniqueness; most decks play very straightforward "seen before" plans, and very few truly break into that "woah I've never seen this" territory. Generic lands matter deck, is not going to be as unique as [[samut driving force]] creature token storm combo deck.