What it is for me is he sets up too inside on most tackles, doesn't show any pop in his hits, and usually tackles so low that runners run through him a bit. Watch how many times he pushes a guy back on a tackle vs. how many times he gets dragged for a yard or two while tackling someone. Generally when he hits someone, they go down, and that's good. He's solid. But for a run-first defender that's just OK in zone coverage (doesn't turn his hips and run fast enough) and loses yardage on a tackle, we could do better.
We could also do much worse, and he's a solid, serviceable defender. We'd all love Patrick Willis back there, but Minter is a good player.