But I have a question - much was made of how the destruction of Deneva was an incredibly big deal. Have we heard of this before? What makes that such an important Federation colony over and above the presumably hundreds of others that were also decimated or destroyed?
Deneva first appeared in the TOS episode "Operation--Annihilate!," and has been referenced in numerous novels since, and was featured perhaps most prominently in Articles of the Federation. And they're not a Federation colony -- they're their own Federation Member world.
Basically, Deneva is sorta like the Rigel colonies -- we haven't really seen it that often, but we always seem to hear about people from there. So it's fair to make a huge deal about it.
As for why Deneva was featured -- well, I'm not David Mack, but I would infer that the narrative particularly focused on the destruction of Deneva both because it's relatively close to the core worlds (thus creating greater fear for the core worlds' fates than would the destruction of, say, Antares) and because, well, he needed to show, in detail, one world being destroyed so as to establish what it was like for all the other worlds without having to stop the narrative for all of them.
There's also the not-inconsequental point that Jasminder Choudhury is a Deneva native...