Song's style is very different to any other DM (position currently debatable) out there. It's like he hasn't heard of normal running, and just kind of trudges along on fast forward.
Maybe that's why some people don't fancy him that much, I dunno. In my opinion we haven't really got anyone like him in the team, which in theory would make him hard to replace but I'm not too sure about that. Looking at his strengths, I suppose he's one of the few players you can count on playing a whole season, and generally speaking is a solid performer. Even if you prefer him when mopping up, you can't deny it's handy having a so called defensive player pop up with an assist or goal increasingly often. For example, John Obi Mikel has no goals in over 150 appearances for Chelsea. Yet, Alex does have his flaws. Sometimes he's guilty of overplaying in simple situations, and is prone to brain farts that are absolutely inexplicable. If he could cut those two habits out he's be that much better, and I think a lot more fans would rate him.
I appreciate the role he plays in this team, but definitely wouldn't mind seeing competition brought in. Think it's not a simple matter criticising his positional play because a lot of that is down to what Wenger asks of him. He's had games where he was absolutely everywhere. Covering for our defence and muscling opposition off the ball.