As someone who first played a CD copy of Half-Life before picking up the Steam version, it's amazing to see that they brought back features which were neglected from the Steam release – most notably the viewroll when moving left & right and a semi-recreation of the old main menu.

Many of the QOL changes are greatly appreciated (especially grenade physics), the changes to technical limitations (and addition of func_vehicle) will be a godsend for modders, and I'm surprisingly over the moon that Valve have acknowledged the psyched out Barney from the alpha.