I think everyone knows the importance of velocity. Especially velocity that is rare.
Nobody wants to hear about the exceptions, but why not, they do exist. The problem when it comes to scouting is this... If you see a guy throwing low 90s, you know he can throw low 90s. Still there are a lot of those types these days, so all the other stuff separates the pitchers.
When you see a guy thowing low or mid 80s and he is performing well, you have to wonder if he can do this consistently against better competition. Then yo have a zillion kids capable of throwing low or mid 80s to compare him to. Are you going to go watch him pitch time and again to really see how good he might be. Is he better than this other low to mid 80s kid? Would your time be spent wiser and employment more secure, going to watch the low 90s guy again? It is a hard sell knowing you might be wrong about a low 90s guy, how do you justify being wrong about a low 80s guy.
It is actually something we can do better than the MLB scouts. Because of what we do we often see that mid 80s pitcher perform several times. It's our job to see him, it is the scouts choice to see him again! So sometimes we see the kid that has special ability and potential despite having below average present fastball velocity.
Most of the low 80s guys I have seen have some success in professional baseball relied on unusual deception, command, or trick pitches. Good example would be Jay Tessmer who is in the University of Miami Hall of Fame. He tried out but was cut from the team his freshman and soph. years. Finally making the roster as a junior. after setting records at Miami and being an All American, he was drafted in the late rounds by the Yankees. He threw sidearm, almost submarine and basically had one very unusual pitch and great command. He set records for saves in the minor leagues and eventually pitched some in the Big leagues. His pitch was a frisby type slider that actually did rise because of his very low release point. Hitters looked helpless trying to hit that pitch. His fastball topped out in the low 80s with great movement and he could paint with it.
Wonder how many young men would accomplish the things he did after getting cut both his first two years in college? Never underestimate the power of persistence!
Anyway there are many exceptions when it comes to those that lack velocity.
I do agree with most everyone here though, velocity is the first separater. But there is much more to consider. If it were just velocity, the hardest thrower each year would be the first pitcher picked.