"When it comes to sketching out melodies, do you have a general criteria for avoiding over-complicating melodies? Is simpler melody almost always better with progressive tunes?"
Honestly.... I don't really think about it all that much. More of an intuition thing really. I usually just play stuff by ear and thats about as complicated as the process gets for me.
"As for sample diving, especially for things like transition FX or drum loops, what makes one sample really better than another?"
Nothing really... again just taste and intuition. Something I try not to over analyze.
"With enough time and processing, can't you get just about any drum sample to sound decent in a mix?"
Sure... to a point, but you'd be wasting your time when you could just find the right sample to begin with.