The colors are great on this, and as a big fan of both Fluttershy and Dr. Horrible, I absolutely love it. The drawing itself is quite life-like, but I think since this is in fact Dr. Adorable and not Dr. Horrible, the colors should be brighter to work better with the brightness of Fluttershy. This piece is very good, and I absolutely love your drawing technique. I love the pair of flies (they look kind of like flies, but if they're something else, feel free to correct me,) I'm also a big fan of the facial structure of FS. There's not much more I have to say about it, all in all I really like it - keep up the good work!
It looks beautiful, and has a perfect color choice, but ponies combined with half life has been done several times in the past . It would've been more exciting if fluttershy was fighting off some zombie ponies rather than parasprites. Despite some flaws, It's very well done, the vines around it are a nice touch, making a more exciting. Fluttershy looks fierce and not afraid of her enemies, which shows that you did well with giving her expression. The suit makes this much more original compared to other hoof-life works like a Gordon Freeman pony. If I were to explain in it one word, it would have to be exciting.
"Half Life is better WITH ponies, but you want to grow combustible lemons" There, I fixed your reply ^-^ . And in all seriousness why hate on this wallpaper? He probably put a lot of effort into it. And for you to take 12.2 seconds to write an insult is pretty rude, sir. If you don't like ponies, fine. Nobody said you had to. But you could just as easily ignore this, and we are all happy. You don't have to look or use this wallpaper. We can if we want. That's it.
The reference is going to be either an experimental Combine soldier, after they raided Celestia's country and kidnapped some pegasi, or it's gonna be a poster hanging on a wall somewhere, a random easter egg for people to find.
Or it may just be this posted on Kleiner's wall :V.