I just don't. 
Whether with movies or games, practically every single prequel ever I can think of has been in absolute detriment to whatever franchise it's prequeling.
 
This brings me to Deus Ex: Human Revolution. I love the Deus Ex franchise. Of course, the 1st most of all, and I'm one of those rare bastards that actually enjoyed Invisible War, even despite it pissing me off on several fronts. So they announced a new Deus Ex, and holy frickin' mountains of poo!

And then I heard it's going to be a prequel. One question comes to mind: Why? Was it really necesssary?
No answer to these questions can ever satisfy me, but that's how it is so I rolled with it thinking OK, we'll see the "mechs" doin' their shit before nano-biomod augmentations were developed, and that could work.
Or, and after seeing the trailer with the dude with a beard this is what I immediately hoped, maybe you'll actually play as Paul Denton, doin' all the shit he did before JC was "activated". I could imagine tons of well done tie-ins to the 1st Deus Ex but still being able to tell it's own story. Heck, I always thought the Dentons were just clones, not actually sexually conceived blood related brothers but "clone brothers", so I wouldn't mind if in Human Revolution they made it so the protagonist was the original Denton from whom they cloned Paul and JC and who was also the 1st subject of cybernetic augmentation.
And then I hear the protagonist's name is Adam Jensen. Goddammit!
Ah well. I'm hoping maybe this dude's the 1st ever cybernetically augmented dude who, turns out, is also from whom they clone the Dentons. Gonna hold out for that possibility, which would make everything allright in my book.
 
One thing I'm just not digging is the "superfuture" looking environments. The original Deus Ex was dirty, smudgy, gritty, but most importantly, architecture and massive technology was pretty "nowadays". I like the black n' gold aesthetic but I'm not digging the superclean, refined, and "holy shit floating city over city" deal. The 1st Deus Ex had an almost apocalyptic vibe, or rather, a "slummy" vibe. So, like in the STAR WARS prequels, the past was cleaner? Well, I guess that works, if actually explained.
 
Lotta possibilities. Lotta potential. So here's hoping this prequel doesn't do what every other prequel does to their respective franchises and totally avoid taking advantage of the core setting.