Some of the material I've come across has implied the the C-in-C of Starfleet has a great deal of authority over exactly how the starfleet is to function without much oversight. Depending on the C-in-C and their inclination as either a scientist or a soldier, the entire makeup of the star fleet's ships and their capabilities can skew either way.
This was lightly touched on in Star Trek VI,
Bill, are we talking about mothballing the Starfleet? Our exploration and scientific programs would be unaffected, but...
So take the Starfleet Gene Roddenberry originally imagined, as the scientific, utopian space exploration fleet, while Nichols starfleet was far more militaristic. Some of us attribute it to the V'Ger incident, but it probably had more to do with rising tensions between the Klingon Empire and the Federation off screen. I'd suggest that this is implied due to how bitterly reluctant most of the crew of the Enterprise is for peace. That the organian disarmament didn't hold, and that everyone lost something due to prolonged conflict with the Klingon Empire. There is sadness and a wounding to how Shatner and Co deliver their lines when discussing the Klingons, and something intense about how Chang views Kirk as a formidable warrior. Perhaps the war with the Klingons went cold after it got hot.
So what does that have to do with this ship? Well, think about the attitudes of the Federation now. Vulcan was destroyed by a ship no one saw coming. Mass genocide. There is a thing, out there, not just one thing, but many things, terrifying things in the galaxy that no one can predict or defend against, and now the only option is for Starfleet to transform from a:
interstellar peace keeping armada
into something far more aggressive and sinister. Something darker. Something deadly.
If this movie is in anyway allegorical the way Star Trek 6 was allegorical, then perhaps when the Federation suffered a great a loss as the United States did on 9/11, it gives itself permission to become whatever it needs to protect itself, even if it violates the prime directive or values of it's own. Perhaps the centerpiece of the real villainy in this film isn't cumberbatch's terrorism, but that Kirk gets punished for violating the prime directive to save spock, when at the heart of Starfleet, the Prime Directive is disregarded for the sake of defending the Federation itself.
Whatever this ship is, it's a symbol, it's own symbol, which is why it resembles nothing iconic we can compare it too. My bloodpressure has been through the rough anytime someone compares it to the Excelsior.
What I think those people are talking about is the feeling you get when you compare, on screen the transition from the Art Deco style Enterprise to the Japanese Neo-Modernism that the original Excelsior had. That when you compare the NuEnterprise to the U.S.S. Holy-Crap that you have that same sort of visceral reaction to their contrast and comparision, but that is a different sort of feeling... a feeling that can be best described as being of the same material, but not the same substance, the same shape, but not the same color (which is as best as I can put it as a graphic artist...)
when you compare and contrast the NCC-1701 vs the NX-2000 you get a feeling that they are different flavors of Starfleet's history. That the romanticism of the Constitution class is giving way to a far more functional and intentional, modular, and practical future. From the point of view of the screenwriter, it should be considered a future within a future.
U.S.S. Excelsior suggested that the Future would be new and different post Enterprise. (From the vantage point of SFS without seeing 4-6)
This ship, is like a grim reaper, and I fear that our efforts to identify it or compare it to things we have seen before diminish it's own unique symbolic significance that this will have to the franchise in the future. Not that it matters. We'll all forget this pre-movie speculation as soon as the credits roll in the theaters.
TLDR; This ship is a symbol for the harbinger of death for the Federation's future, not any class like we've seen before. This ship is starbuck