If you own Half-Life 2 but never bothered with its freebie tech demo, Half-Life 2: Lost Coast, do give it a whirl. Its showcase of HDR lighting might have lost the "Huh, nice" factor it had in 2005, but it’s still a satisfying slice of punchy HL2 combat, with a nod to the original Half-Life that Valve regretted not making in the main game. I know this because Valve told me, via Lost Coast's other innovation: its developer commentary.
I love these things, these spinning speech bubbles inflated with knowledge straight from the FPS coalface. So do Valve, judging by how dev commentary has appeared (in identical, node-activatable form) in most of their games since. They’re interesting and illuminating, don’t interfere with the game unless the player wants them to, and help build design literacy. Why, then, don’t more games offer commentary as well?