I'm going to structure this writeup around the albums which have been released from what I believe to be Miles' most exciting and fertile years (1969 through 1975) : classics like Bitches Brew, A Tribute to Jack Johnson, On The Corner, Get Up With It, and Pangaea.
By the time 1969 rolled around, Miles had been looking towards and striving for new sounds for a while. His famed 60's quintet had been together with only one personnel change for a five year-plus run; this band had consisted of Tony Williams on drums, Herbie Hancock on keyboard, Ron Carter on bass, and Wayne Shorter (who replaced George Coleman in 1964) on saxophone. Five years is a long time for a jazz unit to be together. What once seemed inventive and exciting had probably started to sound like clich to Miles. That band featured great tone and instrumental virtuosity - but by Miles' own account simplicity and directness had been lost.
One can compare the straightforward, soulful reading of Wayne Shorter's “Footprints" done on his own Adam's Apple album (it's also available on Blue Note's excellent The Best of Wayne Shorter) with the version done by Miles' quintet on “Miles Smiles", where the tune becomes a backdrop for the usual pseudo-Spanish tinkling around and theatrical flourishes that characterized that band's sound, at the tune's expense, for an example of this.
Miles had been moving in a simpler, more “modal" (a term that he helped to popularize during the 50's) direction for a while. Miles In The Sky from 1968 started with a brilliant, 16-minutes plus track “Stuff" which cycled and floated in a gentle soulful manner and sounded unlike anything that anyone else was up to at the time. Filles de Killemanjaro from 1968 marked the end of the old quintet, with Chick Corea and Dave Holland coming into the band partway through the album - the music occasionally rumbled and exploded, but was also marked by long, rather lovely modal sections. To me it sounds like ambient jazz. The buzz on this album in retrospect is that Miles was “flirting with rock forms". (This is actually one heck of an album - well worth purchasing. In addition to its other charms, and great playing by all concerned, Chick Corea's lovely, peaceful cycling through the lengthy “Mademoiselle Mabry" is more than worth the price of admission).
Miles continued to flirt with what were certainly different forms, perhaps related to rock, or to soul. His next LP, In A Silent Way, was hailed as a groundbreaking effort although I feel it's a bit overrated. The music was somewhat hypnotic and repetitive. Joe Zawinul and John McLaughlin had been recruited to play on the album, and their presence together with the restraint shown by the other musicians (for once, Tony Williams does not run rampant on the drums - he plays simple “rock" rhythms primarily) yielded what was again a very “ambient" album.
Miles wanted his music to get more basic, more in touch with a blues feeling. In his autobiography he states “See, when I used to listen to Muddy Waters in Chicago down on 33rd and Michigan every Monday when he played there and I would be in town, I knew I had to get some of what he was doing up in my music. You know, the sound of the $1.50 drums and the harmonicas and the two-chord blues". At this point he started to focus in on the more modern and aggressive sounds that would inform the rest of his works. His girlfriend Betty Mabry introduced him to Jimi Hendrix, and the two of them hit it off immediately. Miles appreciated the power in what Jimi was doing, as well as appreciating its grounding in blues and other black forms. Sly Stone and James Brown were also by Miles' account big influences on what was about to become his new sound. Things were about to get a lot more African. “My Funny Valentine" was about to go out the window. 
In August of 1969 Miles assembled numerous massively talented musicians into a New York City studio for the Bitches Brew project. He brought in “musical sketches" moreso than tunes - as he had 10 years previously during the Kind Of Blue sessions. The musicians would jam on themes according to Miles' direction (during three “all-day" sessions), and the jams would be edited into pieces. It was an abstract way of working, a bit different than anything done previously by an artist with commercial viability - the tape recorder would deliberately be used, in “artistic" fashion, to shape the pieces after the fact. Hence musicians could explore ideas at length, without a burden of knowing that everything that they played during a “take" would necessarily be presented to the public with their name on it.
What makes the album superb is the playing. The music swings gently, in multiple directions at once. It is a new kind of swing. Jack DeJohnette and the other drummers on this recording deserve a world of credit for their subtle, tugging playing.
Multiple electric keyboards, usually two per track, swing and swagger across this musical landscape (Chick Corea, Joe Zawinul, and Larry Young are at the keys). John McLaughlin contributes electric guitar playing which is occasionally possessed of brilliance. Bennie Maupin's bass clarinet, Wayne Shorter's sax, Airto's percussion, and the basses of Dave Holland and Harvey Brooks also contribute towards the music's tonal palette.
On top of it all we have Miles.
His playing had always been minimalistic, and he had always been comfortable playing blues-based forms. Here he found his most natural expression, and contributed forcefully to the music. He laid down the real stuff, the essence of music, on his trumpet and topped the whole thing off brilliantly.
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