Okay, we've all been chattering and complaining about em briefly here and there, but I think these 3 things are really important issues to address, since they are essentially exploits, but since they're exploited so widely, so much of the game's balance became built on the existence of them, and every sensible player embraces cornershooting, radarshooting and detecting enemies using their sound, and views them not as an exploit, but as a basic tactic.
Therefore I think we cannot go and just remove these "features" for it will seriously screw up not only the balance but also the basic gameplay experience. Instead, the game (not just it's balance, but it's mechanics) should IMO embrace these as features. Here are my suggestions on how this can be effectively realized...
Cornershooting
The 3 main issues here are, 1) you can shoot but not look behind a corner, 2) enemies cannot hit you, 3) afaik you still cannot cornershoot from every angle with a shotgun (which most clearly of all shows how it's more a bug than anything).
What I propose is that when you stand at a corner, you should be able to see what is behind it, and you should also be able to shoot enemies from there no matter in what direction.
Enemies CAN see and hit you when you're standing behind a corner, however, there is a certain chance that they hit the wall instead. This would adopt cornershooting as a feature similar to sidestepping, and one that can be properly balanced.
Sound and radarshooting
Listening to sounds to detect enemies and get an idea of their location also plays a very important tactical role, but it's OP, since it's essentially like a free Int2 where the exact location is not explicitly shown, however the exact location can technically be inferred. To clarify, if for instance an AI were to play this game, sound would be an actual Int2 since it can measure it's sound on each audio channel and logically infer the exact location of any sound source. Ultimately the exactness just boils down to "how well you can listen".
Here, I would suggest that you can only hear the sound of entities that are currently within the character's vision (or intuition) range.
To give this a counterbalance (and because this would almost completely destroy the merit of radarshooting), if an enemy gets hurt within a certain distance beyond your vision range, it's sound should be played, and an intuition-like indicator should appear for 1 turn on the position where the sound was heard. This makes sense too, since cries of pain are louder than random murmurs.
Notice that both of these features have a numerical value to be decided, ie. the cornershooting hit chance and the radarshooting distance, so that would mean that they would be under proper control and they can be precisely balanced out.
Discuss!