I actually have the same question... been wondering this myself, because a few other skills that we have that say they lower our damage actually make it fairly clear (ie. Stiffled rage and the 20% attack rate decrease)...
I also would like to know the % chance to go beserk... at 29, i feel like i have very few skills that increase my chance to go beserk when i'm NOT the main tank, but i'm hesitant to use raging strike because a) I don't notice my damage or attack rate being decreased, but that doesn't mean it's not or by how much... and b) i don't always go into rage when i use this skill, so sometimes it feels useless...
Some clarification would be nice...