The trackpad and the keyboard. But I think the overall sturdiness is what impresses him.
In my experience the keyboard is better, the trackpad comparable, and the anti-reflective screen is much better for almost all purposes in almost all circumstances.
Anecdotal experience suggests they're more robust than Apple hardware too. A colleague killed her MacBook Air with a spilled drink; my L520 has a splash-proof keyboard that has resisted a few good spills.
Likewise, I've cracked the glass on a MacBook Pro (runs right to the edge of the screen; easy to close on a plug or cable) but never so much as scratched the screen on any of my ThinkPads (I've owned a few).
Oh, and the ThinkPad is designed to be user-serviceable too; I wore out the space bar on my L520 and replaced it courtesy a site that sells individual keys for that purpose.
In the aftermath, Apple wanted somewhere around $700 to fix it (motherboard, keyboard, trackpad). I ended up giving it to my very-handy-and-can't-see-any-hardware-die uncle, who found a company that specializes in repairing liquid soaked equipment. Only ended up costing him about $200 to fix it, but many hours of research and re-assembly.