At the risk that we should get any leakage from the 'can accompanists lift the music' thread, I think for me a really good tune works on a variety of instruments and in a variety of arrangements.

I'm with GUEST...Wrestler, it's the slight quirk: the transitory confounding of expectation that makes perfect sense within the overall arc and structure of the melody, but can't necessarily be predicted on very first hearing, and that same effect then sticks with you thereafter and (for me) makes me want to learn and remember the tune.

Doesn't matter whether it's a subtle shift of the rhythm, or a particular interval in a particular place, or a temporary diversion into a different mode or key ...