I'm no expert, but Diggs has nowhere the explosiveness that Harvin and Deshaun Jackson had/have, nor the top speed that Jackson and Ted Ginn had/have. Not saying he won't be great, but he doesn't have ultra elite explosiveness (think Anthony Gonzalez) or top gear that says "gone" in a college game once those guys got daylight.
Plenty of hall-of-famers didn't have it either, so I'm not knocking Diggs at all, but I hate these comparisons to the most explosive and very fastest people in the NFL. I know it'd be less fun, but I think it'd be a lot more realistic.
Really hope we get him, though. Kid is a player, no doubt about it.