Official expansion talent previews

[Xanthix]

Slam suffers from the limitations of being a "cast" spell: it requires the warrior not to move, it can be interrupted by damage, and even if you improve it with talents its subject to global spell cooldown.

Improved Slam probably works like the Flash Heal cast time reduction of the prophecy set. The bonus reduces Flash Heal's cast time to 1.4 seconds, AND reduces the global cooldown Flash Heal causes to 1.4 seconds. Improved Slam may reduce the global cooldown Slam causes in such a way that you can still spam Slam.

However Improved Slam is a 21-point talent and so you cannot have it and Endless Rage.

And in my personal opinion, anyone who thinks that Warriors are going to dominate because of using Endless Rage to spam SLAM of all things, has never tried to use Slam in PvP before.