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.
If you still have the serial from that CD copy, you can plug it into Steam and redeem a copy (and also the expansions, depending on which version you bought).
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.