Basically, it boils down to this.
If you asked every GM who the best S in the league is, you wouldn't get a unanimous decision. If you asked GMs to list the top 10 Ss in the league, Roy Williams would be on quite a few lists (how many, I do not know).
What this means, most likely, is that there really isn't a consensus amongst GMs/scouts/etc on player evaluations. Sure, there might be an broad agreement such as "Randy Moss is one of the best WRs in the game"...but when it comes to ranking players, you will get diverse opinions.
Yet... when you go across message boards, entertainment news sites, etc, everyone wants a ranking. So people like us, with significantly LESS knowledge than front offices, quibble over such pointless arguments because, hey, we are special.
Somewhere along the path, when enough people say it to form somewhat of a consensus, it becomes fact. The truth rarely matters; perception does.
Hence, Roy now sucks.
The most likely truth here is that on the vast innerwebs, there are maybe a handful of people who can fairly accurately judge a players skill.
Somewhere out there, is the correct answer to "How Does Roy rank compared to other Ss." There is an answer to it.
Its just that 99.9% of the world doesn't have the answer to it.
(Edit: this doens't mean we should avoid discussing stuff like this. We should engage in it if we want to. That's the point behind a message board.)