What does he do that is special? He is slow and can't block. I have heard he has "elite" hands on this board, but that doesn't really matter if you don't have some wiggle to get open. Witten is slow but he is always open. Maybe Witten can mentor him.
He has outstanding ball skills, meaning he locates the ball with ease and attacks it at its highest point. Being 6'6 with that skill set and excellent hands makes him a very dangerous (and needed) red zone threat right away. Everything else in his game can use development which is fine as long as he can contribute inside the 20.
also he isn't slow as you put it. He ran a 4.8 forty but i wouldn't be too concerned with that number because TEs very rarely run 40 yards on a straight line in a game situation. He is a good athlete and will be more improve the more he plays. His blocking technique isn't what his main problem is, his main problem is his strength at the NFL level, but as he gets stronger he will become a more complete TE.
He's going to be a match up problem because he will be more athletic than LBs and too big for DBs.