Just wondering what specifically was different between that and the regular gameplay mode ?

The HUD in hardcore is taken away, plus it takes only about 2 or 3 gunshots to die

It tries to make it more realistic, just a few shots and you're dead, no HUD as Redbullet mentioned, no grenade indicators - you here the tink sound when one is nearby.  I've preferred this mode all the way back to COD4.

You can complete the game on other modes

No killcam either and friendly fire by looks of things... I like it.

Visible penetration.

One thing I have found in my brief time playing hardcore is the camping, especially on the large, open maps because without a killcam they can hide. But it adds to the challenge so it's no biggie.

Hardcore is the only way to play no matter what game mode, though It's very hard to suggest until you learn all the maps or you will be very lost without a HUD.

hardcore is a harder way to play online. the game changes a little like u don't have the HUD anymore, it's easier to die, it's really it for what the difference is.

its the better mode, and thats all you need to know

It's only a better mode for people who like to shoot 1 bullet and the enemy dies...

With hardcore there are no unbalanced weapons, they all kill in a couple of shots.