actually it's even far more common than this thread suggests, here is a pretty extensive list of examples. (TVtropes disclaimer: do not open this link unless you're willing to lose several days in a single wiki).
Since there's already been so many examples concerning visual feedback and controls, how about a little sound-screw? Of course this requires you to have somewhat meaningful audio to begin with, but then there are quite a few nice possibilities (and they are a little less-known than most of the above). for example:
- all the wonderful things ServantoftheLord suggested for health-bar and damage indicators can also be done with audible indicators (groaning when you take damage, LoZ's low-health beeping noise)
- switching stereo-channels.
- footsteps/breathing/monster-sounds behind the player (extra-evil since it can train players to ignore those, resulting in them being attacked by monsters from behind even after the condition has worn off)
- The swapping allies-enemies could be split into two: one only swaps the appearances, the other only the sound effects. So even if the player knows he's being affected by a condition, he still doesn't know whether he should trust the visual or the audible indicators.
- Also, one not-audio suggestion: if the conditions can have different "levels", how about adding not only swapping left-right controls, but also the much harder 90°-turn (WASD -> DWAS)
Hope those where somewhat useful,