The point that is ostensibly up for debate is whether the movie is culturally significant. There can also be debate as to the extent, or what culture or cultures lie within its scope, or what qualities or types of significance it may or may not have.
Culture is a matter of social context, meaning its more than just what's within a work that determines significance, but where it fits in a context of intellectual creations, current/historical beliefs, and practices. A work could itself have effects that make it consequential, or perhaps its existence serves as milestone for something else not yet measured or expressed. Sometimes, just being noticed broadly enough can indicate something is at least novel even if it's not world-changing.
To an extent, there is also some arbitrariness as to what is broadly noticed or effective enough to go from individual to collective status or awareness, and some things can have impacts within a discipline or subset of people that may not be universal yet still reveal something about a subset of people (some of the debate can be whether it's possible to be significant across everything).
What ought to be significant isn't as much the question, because humans as a collective can be capricious and unpredictable as to what catches on or what lasts. Things that are perceived as silly, profound, good, bad, or just plain first/last can be noted for causing changes in what is collectively maintained or generated by a culture, or serving as a signpost for when humans try to make sense of events later.
The further collective perception or belief is from what ought to be, the more things that in a better world would be the norm are likely to be noteworthy. Since people presumably have different ideas of how things should be, how that dispute crops up can be illuminating as well.
Even that might not be necessary, either factual basis, or a utopia.
People are myth-making, where it's more important that things be put together in a story that purports to explain things, even if it's a mixture of different sources or fabrication. Not every tradition even claims that its narrative is close to perfection.
Some of the fatalistic elements of Norse mythology that underpin some of the MCU, or perhaps more consequentially for western literature Tolkein's mythos promise no good ending.
I see that as being potentially problematic. However, I saw someone make a personal observation about the representation or heroes they identified with, even if it crossed cultures or worlds. Then they described how it felt see a protagonist or hero that looked like them for the first time.
Then perhaps, what it might feel like for that hero that looks like you being accepted by different people or looked at as something aspirational--who you'd expected wouldn't care for someone who looked like you.
It might not be a rational impulse, but I found the question about whether everyone deserved to know what it's like to have a hero that was like them an interesting one.
That many black reviewers discussed their anxiety as to what red flags or conventional wisdom Hollywood or fans might see as confirmed if the movie, its source material, its cast, its director, did fall apart or failed to perform shows that there is likely a consistent desire for a broadly accepted vision of a black hero in a modern mythical pantheon. That such an element apparently is also accepted so willingly can also be a consequential thing, or a bellwether.
In a broader debate of what should or should not be, that Black Panther from a historical context is not really apologetic about giving an ethnocentric futuristic ideal as something worthy of aspiration may be a point worthy of debate against visions of a post-racial humanity such as in Star Trek.
I'm now interested in seeing the movie to see how strongly that bears out. Some of the reviewed weaknesses and Marvel-movie skeleton may point to a little more stacking of the deck or less idealistic choices for the story or marketing.
However, even if there's an element of social manipulation to it, that it's so broadly successful could be noteworthy in and of itself.
