Set necks have more sustain than bolt-ons. This seems believable for many because they think glue is better than screws. But it might be because Les Pauls are set necks and Strats are bolt-ons, and the ones they have tried seem to follow this. But it's bunk. You can find many guitars that are bolt-on that can be shown to be attached tighter than set neck. Sustain is more than just neck attachment.
An electric that sounds good acoustically will sound good through an amp. Absolute crap. Acoustic sound is not from the strings, but from the strings vibrating the saddles and bridges, then radiating that through the parts. Through an amp is the sound from the pickups which don't detect acoustic sound, but the string vibration interrupting a magnetic field. This is completely different. I have guitars that sound loud and great acoustically, but the pickups are not that great, so it doesn't sound quite as good. And vice versa. There is very little correlation because the physics is completely different.
Alnico magnets are better than ceramic. Another myth. Probably because ceramics are cheaper. But done properly, both can get a great sound. Ceramics do have higher output, so might be better for modern and metal.
Passive are better than active pickups. This is also junk. It depends on what tone you want. Actives actually are more accurate in string vibrations, with higher frequency bandwidth and less string pull. It will be more accurate in the string vibration. But does that mean that's the tone you want? No? In fact, your guitar could be producing tones that aren't what you want, and the actives are reproducing that. And it isn't what you want. But use passives, which may have a lot more tonal color and filtering, and all of a sudden, the guitar sounds better. The pickup could actually be subtracting EQ and not being flat.
Stainless steel frets have more treble and zing. This is debatable. I bet few people can hear any difference. But the longevity of SS frets is clearly better, and thus you see the industry moving towards these quickly.
Guitar woods make a huge difference. I think they make a difference, but not more than the luthier's craftsmanship and construction. It's much more apparent on acoustics, much less so on electrics. And even less so if you play high gain. Proof? Eric Johnson's Cliffs of Dover uses two guitars -- a vintage Strat and an ES-335. These guitars could not be more different in style, construction and clean sound. Yet can you tell that these 2 guitars were used in the song's solo? The clean tone -- yes, you can tell it's a Strat, The dirty and lead tones -- hard to tell. Because the signal chain and amp contribute much more to the tone.
Need to use thick gauge strings to get good tone. This may have come from SRV and his use of 013s -- his clean tone is definitely great. But many use 010s and lower and great awesome tone.
Also -- if a nitro finish is so good and allows the electric guitar tone to "breathe" -- does that mean that all the guitars out there with solid finishes sound like crap? Some of these guitars have pretty thick poly finishes and all sorts of other kinds of finishes. And many of them sound much better than nitro guitars.
Basically the finish on an electric guitar contributes very little to the tone versus other components.