#1
Greetings,
I have 2 late 80's model Ibanez RG550's. It's nice, but I think they need"something".
I've played another Ibanez RG just like mine that a friend has, I think it was all stock. Mine have Duncan pickups. This other RG was soo easy to play, I was able to hit all the harder harmonics (2nd fret..B and D string) and effortlessly got those screaming high harmonics all day long and sustained for a long time. Then , I played another Music Man LUKE guitar a while ago and got the same type of results as the other Ibanez. I thought it was 22 vs 24 frets, but the other Ibanez is 24 so it's not that. I've had them professionally set up before, but mine still give me a hard time hitting those 2 fret harmonics and never sustains for long as the other ones do.
Some guitars that I've tried just in your average music store have this "Magic" and scream so easily while others don't have it. What could it be?
Granted there could be a difference of string gauge or pickups between these guitars but would that make a difference? There has to be something there that could be corrected...minus selling the guitar and buying the "Magic" ones.
#2
Different string gauges result in different sounds for the guitar itself. Thicker strings resonate more but are harder to play, whereas thinner strings have more technicality, but are prone to snapping and tuning too much. Pickups resonate differently too. It's basically how everything is set up on your guitars, is at least what it sounds to me. Different guitars, different setups.
#3
It's a bit of everything. Strings, setup, hardware, pickups, tonewood resonance... all of it. You can make the most out of a guitar with the right setup but a dead guitar sometimes will not respond to changes.

Find a guitar with "the magic" and play it often.