Many coaches feel that any off-speed pitch that you can command for strikes, that is thrown with your usual fastball mechanics and body speed, and travels about 10 - 12 mph off of your fastball speed, will satisfy the criteria for an outstanding change-up.
I agree with you that some movement can also be very helpful on a change-up pitch, but the factors mentioned above: Command, indistinguishable from your fastball, and 10-12 mph off your fastball, are probably more important.
Part of the problem many pitchers have with traditional change-up grips, like the circle-change, the C-change, palm ball: If you throw those pitches with your normal mechanics and body speed and release them with the palm of your hand directed to the target (just like a fastball, right?) and if there are three fingers directly behind the ball (instead of two, as in a normal FB) then you probably don’t take enough speed off of the ball to make it 10-12 mph slower than your FB. If it’s a straight pitch, because of your release, and it is only 4 -6 mph off of your fastball…that’s not so much a change-up as it is a mediocre fastball. Those tend to get hit hard.
According to some pretty experienced pitchers/coaches, the best way to take speed off of a traditional change-up grip (again, we’re only talking palm ball, circle change, and C-change here…) is to pre-set your forearm, wrist, and hand with some pronation, and release the pitch with pronation. Because the palm and fingers of your hand are no longer directly behind the ball at release when you pronate those pitches, you automatically take off some speed, even when your delivery mechanics and body speed say “fastball” to the hitter. The extra benefit of pronating those change-ups at the release point is that you will also get some screwball-like movement from them–that comes from the type of spin you impart to the ball when you pronate at release point.
The down-side is: It takes a lot of work and a lot of quality reps to learn how to release a change-up pitch with pronation and develop it into something you can throw with command at any time in the count.
The splitter is a very nice alternative for guys who just cannot seem to make the traditional change-ups work for them. It is released palm-forward, just like a fastball, so there is no tricky release-point pronation to control. The index and middle fingers are split, of course, so there is not much behind the ball, even when you throw it with full fastball mechanics–for many guys, that results in a very good change-up. Again, you need to command this pitch, and you need to be able to throw it so that it looks to the hitter like a fastball strike in the low part of the zone…a good splitter will drop below the strike zone after the hitter has already put his swing in motion for something he thinks is your fastball.
I don’t know anything about knuckle-curves, I’m afraid. There are certainly some pitchers who have good ones, but I just don’t have a feel for how much is involved in learning one, or whether they can be controlled as readily as the other change-up alternatives. Maybe someone else here at LTP knows the details about k-c’s.
Dusty’s succinct advice should be re-read, though: You need to command a fastball, or you’ve got nothing to work from. From there, you need an effective breaking pitch and a single type of change-up.