What Walker has that sets him apart is pace and physicality.
I know that sounds like an insult, but that's what he's got which makes him crucial to how we play and almost undroppable. We can leave Walker +1 other at the back on the halfway line or higher, and never worry about balls in behind or over the top.
If you cast your mind back to Pep's first season, we got fucked on the break so many times. We'd be dominating a game completely, and then a long ball over the top or 1 decent pass in behind and we'd be in serious trouble. Bringing Walker in and killing that immediately was worth about 10 points a season over Zabaleta/Sagna/Navas.
Running towards his own goal in space, I don't think there's a better defender out there, he's brilliant at getting between the ball and the forward, or just knocking them off the ball and mopping up.
The rest of his game is good enough but nothing special. He's rarely a threat going forwards and he's the weakest on the ball in the team, his defending in the box is usually decent but a bit nervy and Lineker wasn't a million miles off when he said he couldn't cross.
It doesn't really matter though, he's undroppable because of the way he allows us to play so high up.