For what its worth, I heard Hendrix used to literally chain-smoke weed. Also, allegedly there was quite a bit of smoking going on in the studio during his famous recording of "Voodoo Chile, a slight return". Anyway, rumors aside, I think Hendrix really had two things going for him:
First of all, for his time he was undoubtedly the most technically proficient rock guitar player out there. He really was the direct culmination of a long tradition of American blues/R&B/rock, and unlike most of his British or even American contemporaries, he "apprenticed" for five years on the road with BB King, Little Richard, and the Isley brothers, literally the epitome of American Rhythm and Blues music. While Brits like Clapton, Keith Richards, and the like were cutting their teeth sitting in their apartments copying American blues licks from records, Hendrix was standing right there NEXT TO BB KING seeing it done every night! By the time he became a public personality in his own right, Hendrix had literally already been playing with true American legends for five years.
Next, he wasn't just a great guitar player, in the sense of being a good musician. He was also a consummate SHOWMAN. Arguably, playing behind his back, with his teeth, lighting the guitar on fire, had nothing whatever to do with the quality of the music, but it definitely did help build audiences and cement his reputation. He had quite a charismatic personality, and that came across both in personal interviews, and in his playing.
For Brick top, Hendrix was left handed, and though he is most associated with playing ordinary right-handed Fender Stratocaster guitars upside down, these were invariably strung for left-handed play with the thick strings on top. In other words, unlike some left-handed players who adopted idiosyncratic tunings (like Ritchie Havens) Hendrix was effectively playing in the normal EADGBE tuning with his strings arranged in a normal configuration the same as virtually everyone else. That aspect of his playing wasn't extraordinary; he just adopted it out of convenience since the models of guitars he usually favored simply weren't available in left handed from in the late 1960s.
It is true that playing his guitar upside down would have affected his tone to some extent, as well as the "feel" of his guitar when bending strings and when applying manual tremolo, but these are relatively minor differences. Any right handed "could" emulate them with a simple reverse headstock guitar and/or a reverse-leaning bridge pickup, but speaking as someone who actually owns a reverse headstock guitar, I think this is mostly about appearance. Having the tremolo arm up on top of the bridge (rather than normally on the bottom) definitely did affect the way he used it, but again, a relatively minor thing in the scheme of things. Don't forget that Hendrix also sometimes played Gibson guitars, which don't have slanted pickups OR six-in-a-row tuners. Turn one upside down, and its effectively the same as one right-side up, except for the control locations.
One aspect of Hendrix's playing not often commented on is that the man simply had physically large hands and long fingers. That made it possible for him to play bass lines with this THUMB hanging over the neck of the guitar while simultaneously playing rhythm and/or lead parts with his other fingers on the treble strings. The prototypical example of this would be his work in "Little Wing" where he does bass/broken chord/lead licks over each chord in various positions on the neck. Anyway, I think this particular "trick" contributed more to his distinctive style, then the bit from upside down guitars. You can get a sense of this maneuver here:
