Marvin Harrison Jr has the ideal build for an NFL WR; he is tall and slender, with well-distributed muscle mass and adequate power, which could be better utilized if he had a more consistently aggressive mindset and a few more pounds of muscle mass. Several times he was significantly bothered by CBs who managed to physically overpower him (Notre Dame), which should not happen with his frame.
For his height, Harrison Jr is a good athlete, but I do not see the often-mentioned athletic traits when discussing him. He has very good body control and excellent agility for his size.

I do not see Marvin Harrison Jr as an elite athlete because his acceleration and speed, especially on the third level, are good but not exceptional. He struggles greatly to create separation with his pure speed, finding himself almost always in contested catch situations on long routes.

He has very light, quick footwork, and is also very smooth, qualifying as a fluid mover. Thanks to his excellent footwork, he is able to produce beautiful explosiveness on his releases. 
He has an already elite and very precise range of releases, even better than some very good veteran WRs in the NFL. He's a nightmare in press coverage for CBs.
His ability to create separation is a problem on long routes, as mentioned earlier, however, I find that he uses his long arms well, but it could be even stronger if he had a true willingness to physically impose himself and if he managed to develop more quickness in his direction changes.
He often uses push-offs to create separation downfield.
He effectively uses double-moves, and his breaks are sharp enough to create separation.

Marvin is a good route-runner, with a fairly well-developed route tree, and we can foresee a quick improvement due to his technical skills and footwork.
He is an elite threat on the first level and very good on the second. The main problem for him is that he has a very high center of gravity coupled with a very straight chin carriage, which gives him difficulties in being a very precise and consistent route-runner. He uses different tempos in his routes well and has good timing to create space, but his lack of explosiveness is noticeable against the press.

Harrison Jr has good IQ against zone coverage, but I found some inconsistencies in this area against strong defenses. When he positions himself well in space, he manages to sit down in his route to give a viable option to his QB or to create off-script options.

His ball skills are not far from being very good; he has a huge catch radius which allows him to attack the ball at the highest catch point but does not always make the effort to catch the ball in difficult areas for him, like low zones, for example. Although he has very good upper body flexibility, he missed some acrobatic catches due, in my opinion and based on his body language, to a slight lack of willingness.
He still has work to do on his catching technique; it's quite inconsistent, and he relies on body catches when we expect much more from a prospect of his level.
His lack of impact in traffic is a problem; there have been many drops, and he has often allowed corners to play the ball. Here again, his mindset needs to change, and he needs to be much more aggressive and gain power to impose his size.

Even though Marvin has a good transition from receiver to runner, with fluid hips, good footwork, quality direction changes, and good vision with the ball in hand, his lack of speed is a problem in yards after the catch; he doesn't have the athletic abilities to be a significant YAC threat.

His lack of power and willingness in run blocking is a problem, although at times he shows a completely different side; again, the mindset needs to mature.

Marvin Harrison Jr is a very rare prospect due to his size, upper body flexibility, and body control, as well as his footwork and route-running abilities. However, he needs to mature quickly and adopt a much more aggressive and dominant mindset if he wants to be a franchise WR.
He has the potential to be versatile and utilized in the slot and as a WR, in addition to being a threat on all three levels of the field (short/intermediate/long), making him an extremely difficult receiver to cover throughout a game.
There's still a lot of room for growth in him.
I evaluate Marvin Harrison Jr as a top 5 talent in this draft.