I can imagine that the way CJ turns around when you press backwards will annoy people as they'll get shot in the back, but I'm sure people's strategies will adapt with time.
If people have the option to switch this off, this would definitely give them an advantage in multiplayer over people who have it switched on, so I'm not sure if an option to switch to classic controls will work for multiplayer. That goes for the "press button to aim" feature too.