I can certainly agree with what you're pointing out. Lots of enemies simply felt like one giant horde, rather than multiple factions each seeking to take down the Inquisition for reasons of their own.
The thing is, while there's still room for improvement, I think a lot of the enemies did have differentiated appearances. The problem, in my eyes, is that Inquisition didn't put much effort into showing us, the audience, this. I recall a trailer for DAI that had the Inquisitor get into close combat with one of the terrors (I think that's what they're called? The spindly creatures). I think that if they'd put more time in showing us who we're up against, by way of cutscene, perhaps, we might have been given enough to let our imaginations do the rest. As it is, from our camera distance, we see enough to know what they are, but not enough to take in any detail and feel any dread or squeamishness (such as what I felt when viewing the Mother in DAO:A for the first time).
The scene with Hespith was wonderfully done. The atmosphere created was creepy as hell. While it's fair to say that the Darkspawn are a whole other bundle of joy different from Cory and his cohorts, there was definitely space to explore some dark and creepy paths. A mission shedding further light on how Cory and his magister buddies got into the Fade before could've had a lot of potential, particularly since that in of itself was a dark moment - I believe hundreds of slaves and 2/3 of the Tevinter Imperium's lyrium supply were required to breach the veil. That at least would have been more in line with the game's themes.
So TL;DR, I think the issue had more to do with how the game mechanics limited what we could see within the game (cutscene and running around wise), rather than the graphics themselves or the artistic direction.