Red . .
I do not know enough about "playing" basketball to "see" the intricacies of bad defense, except through the box score. What you observe makes a lot of sense.
Kidd has lost a few steps and cannot keep up with the young quick guards. I can see that. Felton's defense appears adequate to me. Perhaps I am missing something. JR has looked good to me. Brewer has had a defensive ruputation, but I have seen very little that stands out. Chandler has been inconsistent.
1. Check the enemy PG's box score
2. Notice "how" they score as much as how much they score.
When I see Felton's lack of lateral quickness (his ability or lack there of to move laterally to attempt to stay in front of his man) it is boarderline concerning. Guards getting into the lane at will, especially a near 40 yr old off a leg injury, considering Felton's speed and quickness; it tells me he lacks technique and fundamentals.
Then I notice his penchant for self-induced flopping on every contested lay-up. This tells me he needs work on proper body position and release.
The floater is one weapon to use against taller defenders, and he does. But, another weapon he needs to improve is purposely draw contact, maintain balance and put yourself between the defender and the ball. That way when you lay the ball up, a defender must make contact (foul) in order to block the shot- and he still gets a quality shot up.
Just like on offense he relies too much on TC with the floater/lobs... he relies too much on the help defense to bail him out. If Felton cannot adequately stop his man, he cannot start. See Lin, Holiday, Nash, Nelson, Parker and a slew of others vs Felton.