How much of Jordan Matthews falling out of the first round had to do with this being one of the deepest WR classes ever?
No way to prove it, but I think in most years, he'd have been a rd 1 selection. Using NFL draft position to differentiate WRs' prospects is dubious even within a single year. But it can be totally unfair when comparing between differing years if there's a large discrepancy in class depth.
Unlikely, IMO.
I think players go approximately where their talent dictates regardless of the strength of their position in a given draft class. Like I said elsewhere, when there are lots of elite WR prospects in a draft, lots of WRs go in the first round. When there are no elite WR prospects in a draft, very few WRs go in the first round.
Teams don't sit there at their pick and say, "Well, we need a WR. There's nobody worth this pick at the position, but we'll take on here anyway." Likewise, they don't sit there and say "Wow, there are a lot of good running backs in this draft. Well, since those other teams already took McFadden and Stewart, we probably shouldn't pick Chris Johnson here."
I think you could see a little bit of that effect at a position like QB where you can only have one guy on the field at a time. If you already have Andrew Luck or Aaron Rodgers, there's pretty much nothing that could get you to spend a first round pick on a QB. But WR isn't like QB. You can play 2, 3, 4, or even 5 WRs on the field at the same time. So if you're sitting on the board and Julio Jones is available, you don't pass on him just because you already have Roddy White.
Beyond that, Matthews wasn't even picked that close to the top 32. He was the 42nd pick in the draft. If anyone out there had him rated as a top 20-25 guy, you'd think they would've shown a little more urgency in moving up to get him. Most likely the correct answer is the simplest answer. He didn't fall because his position was deep. He fell because that's where teams had him rated.