Interesting thread. I have always thought that certain keys have a richness to them...Eb is one of my favourites....BUT I am not sure whether this is just in my head and because of the 'feel' of physically playing on certain combinations of white or black notes. (Yep, if you are a piano player, I know!) rather than being the actual sound.
Maybe if I got someone to use the transpose button on my piano...then tried playing it without knowing the number of semitones, changed...I would find out if I have been deluding myself.
When I play for others, I try to find a 'comfortable' key for the vocalist, if possible, or find a compromise between non concert pitch instrument players so that they aren't playing in a key they find difficult. I do think it affects the overall sound of a piece, particularly if you have already got used to hearing it in a certain key. If I play blues piano with many guitarist mates, I have to get in quick or they always make me play in E and they really don't like playing in F, which is my favourite!
Is it the actual physical motion of playing of a tune on your instrument of choice,(where you put your fingers?!) or the sound that you hear that makes the difference...or a combination? Not got a clue, really!