Williams needs to improve his strength. I don't think that he is physical enough to be a top flight WR at this time. With improved strength, he can be. He is very good after the catch, and is deceptively quick. He's also a very good blocker at the WR position.
Street is someone who runs good routes, but is not a particularly explosive athlete. He could also stand to add strength, but it's hard to really know what they have in him due to so few targets.
Harris is the best blocking WR on the team, but looked like he was running in mud the majority of the season. I don't know if he was playing through a leg injury, but he didn't look explosive at all. That being said, his special teams contributions are worth bringing him back.