Each key has a different character. More than once, A major has been referred to as a "sunny" key and that it's no coincidence that Beethoven chose it for his 7th symphony.
Maybe before the advent of even-tempered tuning certain keys had an individual character and composers chose to write in these keys for this character. Now days, this isn't the case, but the tradition and romantic notion still prevails.
If A major sounds all that different than Bb major then:
1) You have perfect pitch or
2) Your piano isn't tuned properly
It is register and the music itself that is going to make something sound "sunny" or not. Not the key it's in. Not today.
Certain patterns of music fall under the fingers differently based on the key. It's these patterns that influence today's writer more than anything else.
These are my opinions only. (As I don my NOMEX!)