Mariota is very difficult to project as an NFL player because he just plays a completely different game than he will have to play in the NFL.
He plays backyard football where he breaks contain and becomes a threat to run or throw, and either finds a wide open receiver or runs for a chunk of yards (and he's great at it and a lot of fun to watch).
But you almost never see him sit comfortably in the pocket and go through his progressions, or make a nice anticipation throw. When teams are able to contain him in the pocket, he looks very uncomfortable, and tends to get sacked and/or fumble. 31 sacks taken last year was way too many for a guy with such good size and athleticism playing with so much talent around him.
Then again, he has phenomenal athleticism, good size and strength, a good (not great) arm, and good intangibles.
I think Donovan McNabb was similar coming out of Syracuse, and he adapted and had success in the NFL, but I knew at the time McNabb was extremely smart so I was pretty high on him.
All that being said, I wouldn't draft Mariota #7 overall unless I was sure that he's extremely intelligent. He will have a lot of learning and adjusting to do. If doesn't, he's probably going to bust out of the NFL. He's also not a day 1 rookie starter unless you plan on going 2-14.
On a totally tangential note, I thought the other QB from Oregon (Mannion from Oregon St) threw the ball better than Mariota at the combine (then again, Mannion's 40 yard dash time was like an hour and a half compared to Mariota's 4.52).
