And how do you define command? Throwing strikes? Not throwing the ball down the middle? Not throwing non-competitive pitches?
Yes, those are 2 very important pieces of that puzzle. But that is only the 1st step (at least in the big leagues). The true hallmark of command is the ability to put the ball into a certain portion of the strike zone, consistently, time and time again, regardless of what pitch is being thrown, and wether or not you are pitching from the windup or the stretch.
If I had to quantify it, I would say that I tend to think of it this way:
If we define the strike zone as a rectangular area, let's divide that area into 4 equal quadrants. Then let's number those quadrants 1 thru 4 in a clockwise manner, so that quadrant 1 would be generally be defined as "up and in" to a LH hitter, w/ 2 being "up and in" to a RH hitter, and so on... MOST big league pitchers w/ average or better command can consistently throw their primary FB with intent, to any one of those 4 quadrants, in game situations, and hit that quadrant 8x out of 10...
and to harken back to your point about not throwing the ball down the middle, and this point is KEY - in those instances where they miss (2x out of 10), the ball stays away from the fat part of the plate. If the pitch misses, it's not in a spot where it's gonna get clobbered by the majority of big league hitters.
Now, let's apply those same observations to the secondary and tertiary pitches: The average big leaguer can generally command their 2nd-best pitch in the same manner, I'd say about 6x out of 10 (again, w/out leaving it way up in the hitter's eyes in the middle of the plate). And maybe the 3rd best pitch is spotted in the same manner something close to 4 or 5x out of 10 (which is why most big league pitchers tend to only use that 3rd pitch as more of a timing disruptor).
Now, let's talk about those pitchers w/ TRULY elite command, those guys that are clearly on another level...
Those guys can divide the plate into 9 equal parts, and put the ball where they want it, in the same exact way I've described above.
These guys in bullpen side sessions can throw 12 FB in a row, w/ a pattern of location something along the lines of " 1 - 9 - 9 - 1 ... 3 - 7 - 7 - 3 ... 4 - 6 - 6 - 4" etc...
Furthermore, they can consistently bury their breaking ball in the dirt on a 0-2 pitch, putting it in a spot where the vast majority of hitters will absolutely NOT do damage (unless those hitters happen to go by the name Vlad, or Ichiro... whom you typically can't base the majority of your gameplan around anyways). They are able to make it miss just enough that it absolutely looks like a lower-portion-of-the-strike-zone pitch to the hitter for those 1st 60ft, getting the hitter to commit to that forward weight shift, and producing weak contact (if contact happens at all).
Similarly, they are able to go up above the strike zone with that high FB, just enough out of reach to elicit a swing, but up above the hands enough so that the pitch isn't going to get hammered. When true command guys throw that pitch w/ 2 strikes, the hitter generally bites (and misses).
When lesser pitchers w/ decent (but not great) command throw that pitch, they generally miss a little too high. The hitter doesn't tend to bite often enough. Same with the breaking ball - they'll bounce it, but bounce it just far enough away from the strike zone that the hitter doesn't chase. And if a runner is on 3rd base in a tie game pretty late, the hitter absolutely knows that the pitcher will be far more reluctant to throw that breaking ball in the dirt 0-2, or 1-2, for fear of it getting away from the catcher. They can sit dead-red in those instances, because they are pretty sure they can eliminate the secondary in that situation...
So, to put a bowtie on all this (yay!)... Missing the fat part of the plate is the start, and like I said at the outset of this epic post, it's extremely important...but the difference between guys that have command and don't are usually pretty easy to see if you pay close attention to the sequencing and pitch counts...
if a guy gets 0-2, does it typically get to 2-2 or 3-2 (like it often does w/ Folty when he's not going well) ... or does the pitcher put that hitter away with the 3rd or 4th pitch of the AB w/ regularity?
Go look at how many 3-1 counts Greg Maddux had in his entire career, deduct the IBBs, and divide by the total number of hitters faced in his big league career. THAT is elite command. Now go look at the same numbers, but only in 1995... that's how I think about command vs control.