I think the thing that makes him "Great" is that he invented a trademark sound, by trying quirky things like doubling and tripling parts in order for to make a fuller sound. His mixes sounded great in AM radio and Jukeboxes, not always so in stereo i feel. If you look at his list of hit records i think that speaks for itself really. There is a few of his records that i really don't think that the 'wall of sound' worked but then when it did. Take 'Be my baby' by the Ronettes or 'you've lost that loving feeling' by the Righteous Brothers. I didnt really think his work on Let it be (Long and winding road) was that good either, And i didn't like the album version of 'How do you sleep'
Yep, Gummblefish hit it on all cylinders here. Most of his stuff did sound better on AM radio and on mono 45s than they do today. And like any other technique or production style, it doesn't always work well on all pieces. Some songs don't take well to a wall of sound in the same way that some songs don't take well to hard panning or chorused vocals.
And I'd add the Spector mix of "River Deep, Mountain High" by Ike and Tina Turner to the list of where it did work quite well. The way he crescendoes that "wall" somewhwere around the bridge or the end if it still adds several BPM to my heartrate when it happens.
To elaborate on Gumby's first point about inventing a trademark sound, he was really seminal in the use of many different instruments used in ways not typically even considered for a given genre of music, which enlightened and opened up possibilities for other producers and engineers. Even though it sounds entirely different, much of what George Martin did with Sgt. Pepper has it's roots in what Phil Spector did a few years earlier.
And along with his "unconventional" use of instrumentation came the added concept of the "wall" itself. It's main concept of multitracking and overdubbing several different instruments in a way where the whole sounded different - sometimes greater, sometimes not, but at least different - than the sum of it's parts, and the ability to combine them in a way that had an eye towards filling the spectral dimemsion rather than just fulfilling an individual arrangement.
These were pretty radical ideas back in the early 60s when the standard bearers were Bing Crosby, Elvis Presley, and the early Beatles, which were all quite formulaic and stale in their production style at that time, and it helped blow open the door for the development of everything new from the Who's rock operas to the orchestral rock of the Moody Blues (both of which started out like the Beatles, as very basic blues and skipple cover bands) to concept albums like Pet Sounds and Sgt. Pepper, to the phychedelia of the late 60s, to the heavy rock of Led Zepplin and it's metal decendants.
Did Spector hit a home run every time? Let's just say his aim with a gun was better than his aim with a fader. But when he was on target, man, it was straight through the heart.
And like when Picasso came along with his cubist-lke perspective art, did everybody find it any more pleasing to look at than some people find the wall of sound pleasing to listen to? No, neither one is everybody's cup of tea. But both were equally seminal and influential to the future of their respective arts.
G.