His reading of the game, both defensively and going forward.
Most midfielders have one or the other, he has both to a fairly high level. Plenty of midfielders will be better going forward, plenty will be better defensively, but he can play both to a high level.
I don't want an out and out defensive midfielder in there, I think it's needlessly damaging to our style of play. An out and out attacking midfielder would obviously be suicide. Dier is a good fit for both.
Yeah I can see that, I just think technically and physically he's pretty average, and I worry that limits his potential.
I also worry that constantly changing position harms his development, though I'm sure many will disagree (including him, judging by what he says!)