What tropes are you referring to?
A lot of the Final Fantasy main heroines have been primarily white magic users and have used a staff, or any other variants of a wand, as a weapon. As such, they tend to be physically weak and are used in gameplay (or in some cases in the narrative) simply to support the main male hero and his ambitions. A lot of these heroines tend to be very feminine in how they move, while physically more-abled female side-kicks tend to be butch, doting, or quiet (think Lightning, who is not a sidekick, Ashe, Freya). Peppy and feminine side-kicks tended to be a ditz as well (think Rikku, Selphie, and even Yuffie)
Stella stands out initially since while she uses magic (thanks to a near-death experience, similar to Noctis), her main weapon is a fleuret (sword, not a variant of a wand). Gameplay-wise, it would have been interesting to see how they represent fencing in-game (whether or not we get to control her, or simpy fight against her). Having a fencing weapon is also pretty loaded choice, as the sport was popularized by (male) aristocrats, so that MIGHT show Stella's aristocratic (and most likely, educated) background.
Despite that, the party scene shows that while she is diplomatic when it comes to addressing Noctis, she is still very feminine with her body language (which oddly, a lot of people take against her). So already, she betrays a lot of the peppy-but-ditzy or strong-but-butch archetypes Final Fantasy has for its female side-kicks. As a heroine, she also occupies a different narrative space as other FF heroines. She is always shown in white to contrast Noctis' black, and we know eventually that she and Noctis had to fight eventually. A heroine with a different and opposing goal with the hero would have been such an interesting and different concept to play around with (again, execution is another matter, but that is all moot at this point). IIRC, people initially thought she was one of the main villains in FFV13 when she was revealed during the closed mega theater trailers. People were shocked when Nomura revealed that she was supposed to be Versus' heroine.
So when people say that they are sad that the Stella vs Noctis scene was lost, they weren't sad simply because the scene cut was cool. It was so much more than that.
Again, that does not show that Stella is a strong character, but merely that she stands out and is conceptually different. I think that people who say that Stella is a strong character actually mean that she is just an intriguing concept of a character, and I thought it showed. People weren't just blindly believing Nomura when he said that "Stella would stand out," people already saw that. Compare this to Tabata, who initially said that Luna was a strong character and that we should believe him, when all we've seen (at that point) was Luna sitting down waiting for Noctis to come.
I'm not throwing Luna under the bus here. I think all Stella fans should be openminded to her character, but when subsequent appearances fail to impress (including a movie), it should be understood why Stella fans are upset. But with the game two months away, I can't wait to know more about Luna.
This post has gone on too long, lol. I might have veered offtopic, but I thought I just wanted to give my 2 cents on this whole Luna vs Stella thing.