I think it's a matter of balance, velocity helps but studies have showed that it can be overrated, one study on HRs found that velocity only mattered if it was 95+, otherwise the rate of HRs off FBs between around 89-94 depended on location, as long as a pitcher kept the FB knee high or on the outside edge, that was all that mattered.
Watching pitchers the last few years, here's a few observations:
1) 89 is probably the minimum, and 91 is required to be effective for most pitchers. The reason is separation with other pitches, you want 8-10 MPH between your FB and change/slider, and 15 MPH to a slow curve. You see this with Hellickson, he's much more effective throwing 91 than 89, because it's harder to wait on the change up.
2) deception matters, if a pitcher's motion has his arm away from the body, it's easier to pick up the release point than if he's throwing in front of his body, where you have to pick the ball out of the uniform (especially a white uniform), Lively's FB seems to be harder to pick up than Eickhoff's, for example
3) movement matters, Nola's two seamer is almost like a screwball, it has so much movement. Lively gets some movement. Eickhoff throws a straight FB as does Morgan - and most of the RPs. A straight FB, even at 95+ is just a lot easier to square up on. If you have a four seamer that "rises" and a two seamer that breaks back, it's just a lot harder to hit. One thing I'd want in a pitching coach is one who knows how to teach grips that make a FB move.
4) command matters. Nola when he's on can paint the outside corner against RHs and bust a FB inside against LHs that breaks over the inside part of the plate. Paint the corners and be able to throw a foot inside to move someone off the plate puts the pitcher in control - conversely, if you consistently throw FBs down the center of the plate thigh high your OFs will be looking up a lot.
I watched Seabold in the college WS, I'd say he's 90-93 right now and should add a little more velocity given his frame and age. His FB has almost too much movement, he struggles to command it at times, but like Lively, he's not afraid to attack the plate. As he develops secondary pitches his FB should become more effective.
Nola to me is our most likely #1 starter the next few years. His FB is up to 92-93 and he's starting to recover his command - before he was injured, he had a long stretch where he was pitching close to a #1. When he's on, he can paint the corners with his two seamer, and his curve and changeup have improved. Still needs to work on the four seamer as an out pitch up in the zone, but there's enough there to be a top flight starter if he can improve his consistency.
It's nice to blow people away, but you can be highly effective without averaging 95 MPH:
You do need to be at least 92-93 or so, Quintana, Bumgartner, Hamels, Cueto, Greinke, have all been top pitchers the last few years with that kind of FB.
Keuchel is the only top pitcher in the 90-91 range (Hamels didn't start throwing 92+ until 2014 and Grienke's been under 92 the last few seasons).
So if I'm drafting at the top, I'm looking at guys with the potential to throw 92+ (average, not peak) with command over throwers who hit 95-97.
Now if Sanchez is for real, we've hit the jackpot, 95-98 t101 with control is rare as hen's teeth.
Kilome is more usual for high velocity prospects, he's filling into his body and adding more velocity, but his command is still iffy.
As Tirado shows, there's no guarantee that a guy who can hit 100 will ever learn to command his stuff.
