This situation seems very straightforward: Delta didn't fulfill their product/service guarantee for this specific flight, on this specific route. Period. They don't get a 'hall pass' when some other variable -- whatever it is -- intervenes to prevent them from honoring the (strongly) implied guarantee. That said, they still make individual customers go through the effort to raise the issue and request some sort of compensation...and even then, it's up to them (Delta) to make the first offer and try to address the issue. Delta counts on most people not making an issue of the rare equipment/service/product substitution in the first place...and then they make it even that much more time-consuming and difficult if an affected passenger wants to pursue the issue further. If Delta were a more honorable business, they would more clearly outline what the implied guarantee translates to in those odd circumstances when they can't fulfill their promise--and proactively issue compensation to affected passengers.

I'm amazed at how many folks revert back to this "just be happy your plane didn't crash" excuse/mentality. Wow.