My personal issue with Rampage is that the boost doesn't scale. The only chance to proc (and duration, I think) does. Additionally, I believe it is quite useless in certain boss fights (like.... the ones that don't spawn minions, or rarely spawn them).
On kill procs are always a bit iffy to me. (because of the boss fight issue)
Rage Retaliation has 0 CD (i.e. always counters) at Level 15.
I think Rage Retaliation is weak because +HP/lifesteal/heal aren't as strong, so Rage Retaliation becomes weaker due to characters being unable to tank heavy damage due to those affixes not being up to mark.
That being said, I feel Rage Retaliation shouldn't be restricted to being a melee range counter, and that most builds either get ignore it, or get Lv1, or Lv15. I think it should be tweaked in a way to give people the flexibility to invest some points in it. At the moment I don't think the in between levels are justified enough to invest points into them.
The other classes will have their time, I think the current pace of discussion is good.