I will agree with you on some points. His rebounding has to improve, especially his offensive rebounding. I cut him some slack on defensive rebounding because many times he's out on the perimeter picking up someone else's man. Your right about his needing to get stronger as well. I think his rebounding will improve with added strength. I totally disagree on his ability to guard smaller players on the perimeter. Even if their able to get past him, their shots are still highly contested. He's a very good pursuit defender. In other words, he's quite capable of blocking shots from behind. Which is tricky in the NBA because many ref's tend to call a foul. Especially if your a rookie.
I will say this. Willie is best suited to playing along side another big man like Cousins or Koufos. His strength isn't solo at the center position, especially if he has little weakside help, which is the case when Gay is at the PF position. One of his strengths is running the floor, and so far, the Kings have done little to take advantage of it. Offensively his role seems to be, just stay out of the way. Which many times takes him out of position to rebound. Not an excuse though. He needs to attack the basket for follow up's when someone else takes a shot. He has some weaknesses, and what I find ironic is that your picking on something that's one of his strengths.