It's not so much that this game's quality changed all of a sudden since the Arkham Knight trailer came out. It's that the Arkham Knight trailer pretty much killed all hype for this game. We have just as much information on AK as we do on this game, except that AK is made by a more reputable studio/publisher, is part of a more reputable franchise, and looks better overall. Again, that doesn't change anything about this game, but which game will get more attention in the media in these next two months? Obviously B:AK. Activision's decision to leave most marketing for last minute really backfired in this case. Every piece of news regarding this game will be buried under Batman: Arkham Knight, regardless of how big or small it may be.
The same thing happened with Edge of Time in 2011. Outside of Spider-Man fans, a lot of people were surprised a Spider-Man game even came out that year. Though at least Edge of Time was also a last-minute thing for Beenox; this looks like a bit of a more major project than that.