According to these tweets, Hardline will differ from the previous games by giving you a flat % bonus, meaning that you can obtain your Orbital VSAT with 2 less kills, and the Drone Swarm 3 kills sooner than without Hardline. At the moment, the % bonus appears to be 20%, although this is subject to change as they fine tune the game.
If you play Hardcore mode, since it has no health regeneration, it has been decided that the blood on the screen WILL stay, although a layer will be removed and blood will only remain on the edges of the screen. There will also be no loud breathing.
In private matches, where there is a bomb or bombs involved, there will be an option to disable the bomb plant sounds (and i assume the bomb diffuse sound as well). This is a very important feature in previous games, as the sound of a bomb being diffused was very clear and signaled the enemy that they were vulnerable. The competitive crowd, MLGs and the GameBattlers and many more, have requested this.
Oh Vonderhaar, y u such a jerk?