Most traits don't matter unless the AI is controlling a player, but there are some important ones that you should look for in any card.
QBs:
QB Release: You want the QB to get the ball out fast. Slinger 1 is the best release in the game right now and is the main reason why players like Aaron Rodgers are so expensive.
Sense Pressure: Average or Better. A Paranoid or Trigger Happy Sense Pressure trait will cause a lot more under pressure inaccuracies without the use of an ability like Fearless.
Throws Tight Spiral: Yes. This trait seems to play a part in how fast the release of a quarterback is in Madden. A player with Slinger 1 and Throws Tight Spiral: Yes will have a better overall release than a player with Slinger 1 and Throws Tight Spiral: No.
HBs:
All Catching Traits: Yes. This may be hard to find since not many HBs are feature pass catchers, but you want to have a player who can catch any way possible.
Covers the Ball: Brace for all Hits and Brace for Hard Hitters are the best in this category, meaning that players who have a high hit power rating are countered by the trait most of the time.
Fights for Extra Yards: Yes. This is important primarily for goal line situations and short yardage. It's also nice for your HB to pick up a few extra yards each time they are tackled instead of just going down on the spot.
WRs and TEs:
All Catching Traits: Yes.
Drops Open Passes: No. You don't want your pass catchers to drop a wide open pass.
Covers the Ball: This is the same as HB.
O-line:
High Motor: Yes
Penalty: Disciplined. This should result in a lot less holding penalties.
D-Line (also applies to Pass Rush OLBs):
High Motor: Yes
All Pash Rush Moves: Yes. The more pass rush moves your D-Line is capable of, you are more likely to win block shed attempts.
Penalty: Normal or Better. Roughing the passer calls are a big problem in Madden, so you want disciplined linemen.
Big Hitter: Yes
Strips Ball: Yes. This sometimes leads to missed tackles, but if you get a sack then the chance of a fumble to go along with it is worth the slim chance of breaking the tackle.
CBs:
Plays Ball in Air: Aggressive. Players without this trait are less likely to jump high enough for a swat or an interception. You want aggressive DBs on the field.
FS and SS:
Plays Ball in Air: Aggressive. Same as CBs.
Big Hitter: Yes. These players have some of the highest hit power in the game. They might as well be able to use it.
MLBs (also applies to Run-Stopper OLBs and Pass Coverage OLBs:
Plays ball in air: Aggressive. MLBs aren't all that great at playing balls in the air, but this year they are extra important due to reaction time decrease for DBs lined up at linebacker.
Big hitter: Yes.
Clutch Trait Explanation: https://www.mut.gg/forums/forum/mut-discussion-10/topic/clutch-trait-explained-838/?post=4306#4306
