Jon.W hits the mark then bounces off it again. Key choice is essential for a singer and therefore it does not go without saying. Does the melody suit a 'doh' note in the middle of the voice range, or towards the top or bottom. Get the answer and a good singer may not be able to sing your song without altering the key. The same applies to instruments but with a couple of extra related issues. How easily does that key fall under the fingers for a particular instrument? Does it allow a string player to ring open strings? If it does, the sound will be more lively and robust and better for dance music. If does not, the fingers have to work harder to get expression and this will show in a more controlled type of sound. In a lot of folk music, the fiddler will not be a happy bunny if you are not in D or G or A. But in jazz, the sax, trumpet and clarinet players will be equally unhappy if the tune is not in concert pitch B-flat, E-flat, or F. Seasoned professional musicians will be less fussed.