So if @chester , @Seven and @Robee allow ?
Witsel is an allrounded central midfielder. Suited for a mid 3 in wich he is not a holding midfielder, nor an attacking reference point. he's allrounded. When he's close to the ball, he'll run towards the player in possession to ask the ball, and distributes it with rather short simple passes. His playstyle is to do that, and infiltrate the opponent close to the strikers to find space there. he has a good shot and header, so he's moderately succesfull with it.
He cannot play holding midfielder. He has poor tactical sense and positionning, and is bad at reading the game. He's primarly a short passer + infiltration. Holding mid completely ruins his game.
He cannot play attacking midfielder. his vision and passing is too limited, and his offensive game is to confuse defenders by infiltrating from deep. He's blocked quite a bit by having to remain in that zone. At Belgium for example, we've seen several times when he was instructed to stay deep, and its never working.
I would compare him to Marchisio's role with Pirlo and vidal in the mid3. Difference beeing that Marchisio is significantly superior at everything except headers.