His tackle. But his overall Defensive ability is nowhere near top club level expectation with the likes of Keane, Makelele, Essien, Kante, Vidal, Fernandinho, Viera, De Rossi.
He's pretty much a limited player but from his performance I believe he's a good midfielder in a midtable club only like Everton and Southampton. If he leaves this January, I wish him good luck but given his ability and how limited he is, I don't think he will ever prove himself to be good midfielder in a top club level.