It's been changed because the .dll files and the .exe files have been taken from the CM Patch. Nearly nobody really has the ability to make custom changes to these files.
You should lose nothing major. I noticed just a couple of the new effects missing, like for example the elemental effect on upgraded Skeletal Fortification arrows (Silver mod).
Can't really tell you more as I never played long without the vanilla s2render.dll file as the sped up animations have been one of the things that have been bugging me the most when I tried the CM patch a long time ago.
As for the specifics, it's been talked about a bit near the start of this thread. Flix said this:
I do not believe you will lose anything too critical. Primarily the fixes relating to FX will be lost (the missing DOT FX, buff FX appearing/disappearing on minions, missing elemental FX on energy weapons and all projectiles, missing impact FX on magic staff projectiles, missing fur FX on elites, some FX performance improvements).
Although if you've played with the changed file for a long time I can imagine you might notice some more differences than me if you go to the vanilla one.
Because of this, the s2render.dll file would be probably the hottest adept for looking into changing. If somebody could revert only the animation speed change and keep all the other fixes, it would be awesome