As a violinist, I'd like to share my thoughts on the subject...
Its all too common for a singer to change key because it doesn't fit their vocal range. It's all too easy for a guitarist to capo their guitar to reflect...
What gets lost is that switching keys changes the mood...however slight (or large). By modulating keys on a whim you alter the mood of the piece.
This is a very easy thing to hear on string instruments...because the strings produce different resonances. D major simply rings on a violin because the tonic (d), dominant (a), and subdominant (g) are all open strings...so you'll get a lot of sympathetic vibrations playing through the piece.
Change the key to Db major and suddenly the tonic, dominant, and subdominant of the scale is not on an open string. You'll get a wholly different sound...
Its plainly obvious (and not really a good thing) when you switch a key because of a singer. "Ohh, this is a singer's key..."
Now then, the most powerful thing you could do with scales is to change their type. For example, using a MINOR scale instead of a MAJOR scale.
Another thing is to come up with a good chord progression. You can change keys all you want, but if you are using similar chord progressions within them...the songs will still sound the same.
Try to deviate from just using I, IV, V...and/or add some tasteful color chords.
Finally pay attention to your rhythmic meter, rhythms, and tempos...and use them to suit your work.
Bottom line:
1. Think minor, major, (pentatonic, blues, etc.etc.) scales...
2. Think chord progressions...
3. What key you ultimately choose will affect the color of sound you will get. d major is not db major is not g major, etc.etc.
4. Don't forget about the timing (meter/rhythms) of the music...