In reply to rslifkin :
On a material basis, the valve seats and the valves as well as the exhaust manifold. The other contributor is CAD- so the exhaust ports are much better cooled to keep those head temps down, and then the port shapes as well as the manifold shapes are tweaked so that you can move them back and have them light off quickly but far enough back to keep cool enough. Oh, and the CAD used to make sure the combustion chamber is a good shape to allow a high compression ratio without knocking.
There have also been some advances in catalyst materials to up the limit for damage- but that's going to go on for some time.
