Feel free to disagree with any of this. These are my observations, plus some advice given in these forums from other owners along the way.
What I like about the Defensive Ratings, there are only four of them, and they do exactly what they say they do.
Unlike Hitting or Pitching Ratings, there aren't combinations which can add to or subtract the effectiveness of each other. A player has an 85 Glove, it's an 85 Glove no matter what his Range is.
Range Factors per position are individually dependent on how your team is made up, including your pitching staff, but one thing they actually show is which positions get the most chances, and that can factor into your positional assignments.
Glove and Arm Accuracy correspond to Errors; Range and Arm Strength correspond to Plus or Minus plays.
At SS I want a player with 85+ in all four categories. I will play a weak hitter in this spot. The SS handles the most chances of any position, so over time the difference between an 80 and an 85 Glove is noticeable, and so is the difference between an 85 and 90.
In CF I want 90+ Range and highest Glove possible. Depending on the rest of the team, I have played a weak hitter here, but I have also used a better hitter with Range as low as 80. But I won't go below 80. When you go too low on Range at any position, the players don't even get minus plays, they just start letting more hits drop. You might not even notice it but your pitchers will suffer.
LF, Range and Glove. It's been pointed out that OFs in this game don't make throwing errors. But Arm shows up in plus and minus plays, and the opposition's baserunning. Which leads to...
RF. Arm is more important than Glove. It's why MikeT23 came up with the C in RF strategy, which I have used with my NL team. It answers the question of what to do with a slugging DH or C who can't field a position, a Kyle Schwarber type. The RF handles the least chances per game, usually less than 2. If you put a Catcher/DH out there with terrible Range and Glove, he'll get minus plays, but an amount you can live with, and much less than playing him at 1B which is what some owners try. And Mike also found after much trial and error that success in RF correlated to Arm Strength and Accuracy.
At 3B and 2B all four ratings matter. But someone else... can't remember who, sorry, pointed out that with 2B handling more chances, you can actually profit in plus plays from the stronger arm playing 2B rather than 3B. That's what I do now.
At 1B, more is better. The recommendations suggest 50s across the board. If a player is a great slugger you can go as low as 40s; if you get up to 70s you're talking potential Gold Glove.
I kind of go with the old reason they said Jimmy Johnson revolutionized defense in college football in the 80s. He moved corners to safeties, safeties to linebackers, all to get more speed in the defense. I move baseball players "up" one position in defensive assignments where I can.