Boy, was that a mistake.
Good receivers, imo, need a whole host of qualities.
They need speed, and athleticism along with it, which is body control, and most importantly, agility. Receivers have to be able to change their body position quickly and fluidly to get in the prime position to snare a pass. Randal Williams, for example, is a fine overall athlete but seems to lack this critical ability to revolve his hips and turn to the play instantaneously. (Good free safties have these hips too.) Receivers who don't have this high level of agility need balls perfectly placed to them, and that just doesn't happen often in football, regardless of who is throwing the ball.
They have to be strong enough in the lower body to elevate over defenders, and strong enough in the upper body to maintain balance when catching a ball in traffic, when the ball and defender is zooming in on them at the same time.
Body control is key for them, and that takes considerable strength, if not the kind of strength a defensive tackle has.
Increasingly these days, they have to be bigger than in the past, unless they have exceptional agility or speed.
They have to have excellent hands, very soft, and be able to focus on the ball as it is coming in.
They have to be not just fast, but quick, and able to increase speed when it is required. That's what's meant by being explosive. A lot of guys with track type speed don't make it in the pros because they don't have the right kind of speed - full striding speed that allows for separation.
They also have to be both precise, and calculating with route running and not give in to the tendency to adlib a route. On the other hand, they have to react to what they're given with the match up and the defense and make adjustments accordingly. They have to be smart. They have to know their QB.
They have to be confident, perhaps egoistic, and believe in their ability to break games. They have to be self reliant persoanlities who don't feel threatened "on an island".
I think receivers have to be so many things to be successful, it's amazing we have so many good ones.
PS This was about 1/3 as long as I WANTED to make it.