Thoughts? Again, in general - especially as related to QB situations, as more than almost any other position (even TE) I think WR is affected fairly significantly by their respective QBs. Mike Wallace without Big Ben's deep ball, is not nearly the producer he was - as an example - or Fitz for the more drastic.
I think there was a similar discussion not too long ago. IMO there are a few levels of NFL receiver:
Elite #1 - A player who would excel regardless of situation. Examples: Dez Bryant, Calvin Johnson, Vincent Jackson, Brandon Marshall
Quality Starter - A solid starter capable of producing strong FF numbers with the right opportunity. Examples: Eric Decker, Mike Williams, Steve Johnson
Journeyman - Good enough to stick around the league for a long time, but lacks real FF value. Examples: Kevin Walter, Michael Jenkins, Nate Burleson
Scrubs - Bottom-of-the-roster youngsters who will wash out of the league in short order.
I think the players in the first group will produce independent of QB play. Their numbers will fluctuate a little bit depending on their usage and their supporting cast, but in general they're going to break 65 catches and 1000 yards almost no matter what. And with a serviceable supporting cast they're guaranteed even more.
The players in the fourth group will be worthless regardless of supporting cast. They're just not viable NFL talents.
Where supporting cast has the biggest impact is with the players in groups 2-3. A merely solid starting caliber NFL WR like Mike Williams will be an every week FF starter with good opportunity and essentially worthless without it. I think guys like Lance Moore, Eric Decker, James Jones, Brian Hartline, Stevie Johnson, and (to a much lesser extent) Jordy Nelson and Randall Cobb owe a lot of their FF output to being in the right situation. They wouldn't be able to produce if they didn't have a certain level of talent, but on the other hand you never look at those guys and think they're among the best receivers in the league. Stick any one of them on the Browns, Jets, Vikings, or Raiders and it's tough to imagine him cracking the 1000+ yard barrier.
So I think a key factor is knowing what you're looking at. Honestly, there is a pretty clear pattern with legitimate #1 NFL WRs. Most of them have special height/weight/speed/explosiveness numbers. It seems nearly impossible for someone without the right combination of physical traits to reach that level. However, there are quite a few great players from group 2 who are very ho-hum athletes on paper. Randall Cobb, Eric Decker, Rueben Randle, and Stevie Johnson are very mediocre combine number athletes, but they're also very good football players and if you give them a decent opportunity they will produce.
It's really unlikely that you'll find a group 1 WR on the scrap heap. Those guys tend to get picked in the 1st round of the NFL draft and very few of them will ever hit the waiver wire before they break out in the league. However, there are plenty of potential group 2-3 types out there. What you're hoping for when you grab a Jarrett Boykin or DeVier Posey is that he has enough ability to become one of those guys. There is no magic formula to differentiate between them though. Each case is going to be a judgment call where you have to weigh ability, short term opportunity, and long term opportunity.
Let's not forget that, as much as we like to think we're in control of our teams, there is a lot of luck involved in FF. Randall Cobb would not have been Randall Cobb in 2012 without Greg Jennings going down. Jarrett Boykin would not have any immediate prospects if James Jones and Randall Cobb hadn't gone down. Larry Fitzgerald would be worth a lot more if Arizona had a decent QB. Eric Decker would not be a top 10 FF WR if Peyton Manning hadn't joined the Broncos. You can kinda sorta anticipate some of this stuff, but a lot of it is just random. That's why I think your best bet in dynasty is to rank players mainly based on talent and let the chips fall where they may in terms of opportunity.