There are many reasons why they bust. Usually it's some combination of...
Work Ethic - There are some players who have the talent to succeed, but lack the work ethic and discipline needed to succeed. Charles Rogers and Reggie Williams are great examples. I think they both had the talent to be 1000+ yard receivers in the NFL. Unfortunately, neither of them had the drive. They phoned it in once they got the big money.
Football Skills - Some guys who get picked high just weren't great football players in the first place. Darrius Heyward-Bey, Quincy Morgan, Ted Ginn, Koren Robinson, and Troy Williamson are great examples. None of them are natural receivers. Their production typically hasn't matched their hype (although Morgan had pretty good stats at K State).
Physical Skills - The NFL usually does a pretty good job of filtering out players who don't have the physical skills to thrive. Few of them get picked in the first round, but it does happen. Mike Williams was a first round pick despite a complete lack of burst and quickness. Peter Warrick was a first round pick despite diminutive size and 4.6 speed. Ted Ginn was a first round pick despite a frail frame with poor body proportions.
There are probably only four WRs who have a chance to be first round picks this year (Bryant, Thomas, Tate, and Benn). Do any of them have any of the weaknesses on the above list?
Bryant - Great from a football skills/physical skills standpoint. Work ethic is the only thing that can stop him.
Thomas - Productive with elite physical ability. Should be a success.
Tate - Productive with good workout numbers. Size and hands are concerns, but he ran a 4.3 and dominated in college.
Benn - The obvious bust candidate of the bunch. Had less than 500 receiving yards last season and is the only guy on this list whose production didn't align with his draft grade.
I'd say Benn is the biggest enigma of the bunch. I'm a little worried about Tate's tendency to body catch (football skills) and lack of prototypical size (physical skills), but his low 40 time alleviates some of those concerns. Bryant and Thomas appear to be can't-miss players from a skill standpoint. They will only be derailed by work ethic/character issues.
I agree with a lot of what you said, but I'm not sure if I agree with the application:Bryant - more of an immaturity thing than a work ethic issue. I'm not sure if "bad work ethic" is something you can predict, and even if it was something you could predict, I don't think Bryant has shown any signs of that. I don't see much of a Charles Rogers/Reggie Williams in him, and if anything, he seems to just really want to be in the NFL (hanging out with Sanders, etc.)
Thomas -- how can you not question his "football skills"? You talk about Ginn and DHB (I don't think Koren Robinson is a good example -- he was a very good WR for a stretch), but Thomas didn't even play in a "football offense." Like Benn (more on that below) this fault isn't a personal knock on him (it wasn't his choice what type of offense GT ran) but I think it definitely increases his variance. Playing in an offense that calls 20 straight runs and then one deep pass doesn't do a good job of showing us a guy's football skills. He may end up having great football skills, but I think for now, they're a question mark.
Tate - I've loved Tate for awhile now. I do think his physical skills could be a question, but I think he's pretty safe.
Benn -- sure, his numbers stunk, and yes, that increases his variance. But there are only four words that could shut down a WR more than "Darrelle Revis Nnamdi Asomugha." And they're "Ron Zook Juice Williams." I don't blame Benn for his putrid productivity in 2009, although I think there are other question marks.