What I like about him is that he doesn't just rush into challenges and leave a gap for the opposition to exploit. That's one of the issues I have with the likes of Kanté, Gueye, Matic, Mendy, etc. What Ndidi does differently is that he cuts out the passing lanes and plugs the gaps first before challenging the opposition player on the ball. There have been occasions where he has rushed into challenges and left a gap, but he hasn't done that too frequently. For him to already appreciate his surroundings and be aware of the passing lanes and gaps that exist is a huge indication of the maturity and intelligence in his game.
On top of that, on the ball, he's also quite composed. He possesses the close control that allows him to evade the initial pressing and maintain possession for the team. At the moment, his passing range and vision is still quite limited, but if he continues to work on that part of his game, he can become what Mikel couldn't be: a solid deep-lying playmaker.
However, if he can't become a deep-lying playmaker, he has the energy levels to be a good defensive box-to-box midfielder/destroyer and allow others to dictate the game whilst he maintains his side's possession.