I think it was ridiculous that they made "Warp 10" some transcendental state where you're everywhere at once...as if they somehow "knew" that when they were creating the warp speed scale.
Why wouldn't they know? In the real world, we use the speed of light in much the same way. Or with temperatures, we know that zero Kelvin is the lowest possible, even though we've never achieved it. The scale is built around the theory.
But this is totally different, the jump from 9.9 to 10 is more than just another increase in speed...it's somehow a leap from zooming about really fast to suddenly being everywhere at once. Assuming this was known when the scale was created, then somehow this phenomena is fundamental to warp theory. But as far as I'm aware it's never really been explained *why* this is so. Whatever speed Warp 9.9999 is....why exactly does going anywhere beyond that land you everywhere at once? Let's pretend Warp 1 is the speed of light - what exactly *happens* from a scientific perspective at 10x the speed of light to cause such phenomena? I mean, Picard and crew zipped around to emergencies at Warp 9 all the time, and they weren't even close to being everywhere at once. It just doesn't make very much sense. But, as I said, this is TV and it *is* a good storytelling device, just a slightly boneheaded one. And a difficult one to realise and execute with any sense of realism.
Warp 10 is the name of a defined limit of V=infinity on a logarithmic scale. What this means is that the difference between warp 1 and warp 2 is not the same as the difference between warp 2 and warp 3 - the difference between warp 2 and warp 3 is much, much greater. The higher your warp scale number is, the more any slight increase in your scale number means a greater change in velocity.
So (and I'm making these numbers up entirely), warp 1 is 1X the speed of light, warp 2 is 10 times the speed of light, warp 3 is 250 times the speed of light, etc. Therefore, warp 9 is 100,000 times the speed of light, warp 9.9 is 120,000 times the speed of light, warp 9.97 is 160,000 times the speed of light... and so on. That means when you see one ship has a maximum velocity of warp 9.9, and another has a maximum velocity of warp 9.975, while that seems like that's hardly any faster the second ship is actually significantly quicker.
This is because the velocity increases in relation to warp factors on a curve. This curve approaches, but never reaches, warp 10. 10 is defined as infinity, which you can never reach since infinity is an endless number. So the curve keeps getting closer and closer to 10 on the warp scale, but because the curve never reaches it, it starts running nearly parallel on a plot versus velocity - so you barely change warp factor, but velocity greatly increases.
I'm making it sound way more complicated than it actually is, so I made a quick and dirty Paint version of the plot. Again, numbers are completely random.
The warp scale to velocity relationship is defined by the curve. I have placed a red dot where the curve intersects warp 1, warp 2, etc, up to warp 10 (as well as warp 9.5). You can see the corresponding velocity by checking where the dot is on the X axis. As you can see, warp 1 is shown as about 40, and warp 2 somewhere around 110, a difference of 70 units of velocity (whatever they may be). The difference between warp 9 and warp 9.5, however, is somewhere around 200 units (and not just because I failed to space the units equally). As you can see, the closer you get to the warp 10, the more dramatic this effect is and the more rapidly you accelerate.
This is all a very long way of saying that there isn't some arbitrary speed after which you either are suddenly stopped from accelerating or are everywhere in the universe. Warp 10 is simply arbitrarily defined by people as infinite speed, and you can always go faster and faster and faster after hitting warp 9, you'll just starting incrementing your warp factor by smaller and smaller numbers so that you never reach 10.
(Warp speeds higher than 10, which appeared on TOS, are supposedly from a different method of calculating warp factors, from before warp 10 was set as infinite velocity. Which means when the Enterprise NX-01 travels at warp 5, it is actually travelling at, say, warp 3 on the TNG scale. Apparently, by the future depicted in All Good Things, the warp scale calculation method was changed once more; My assumption is that as ships started commonly traveling at speeds with ridiculous amounts of decimal places after 9 (Warp 9.99765 for example) there was a need to make that warp 9 area roomier to make the warp factor more wieldy. So infinite velocity was redefined as warp 15 or something, and the Enterprise's Warp 13 was something like 9.99985 on the normal TNG scale.)