Don't underestimate the ability of an outfielder to read balls off the bat better than others, good vision, and a solid idea of the area the ball may land. Tracking ability and solid fundamental routes are vital skills. Then,
Don't underestimate athleticism and speed. These talents make the task easier and gives the outfielder the ability to compensate for any mistakes, but more important to get in the optimum throwing position, i.e, not going backwards.
Arm strength is very important, but look at the usage factors. A deep gap shot caught by an outfielder is an out. With a runner on base, he will probably advance anyway due to momentum away from the action. The arm needs to be strong enough on base hits to keep runners from advancing as much as possible.
Many corner outfielders were once centerfielders, who were once shortstops. In other words, usually the best athletes at that given time. When they get funneled up together, you have the makings of a great outfield.
I don't always agree with the scouting emphasis on raw speed because fundamentals and overall skill and athleticism are more important, especially if the burner is a banjo hitter, but......................
It's a bonus when your CF has 6.5 speed, and your corner guys 6.7 speed, and your RF has a cannon.
Players who appear to be running in quicksand, even though they can hit balls a country mile shouldn't be on Gap Patrol.