A champion pack with frozen/nightmarish/jailer/waller can potentially render you unable to do anything. They can keep you CC'ed infinitely. This is bad game design and has nothing to do with gear, spec, or MP level. Look at it from the other side: they introduced CC diminishing returns for monsters such that they can't be CC'ed 100% (but they somehow forgot to nerf CMWW). Why didn't players get diminishing returns?
Ever encountered two frozen packs at the same time? GG. One of the reasons my main characters stay softcore. Unless there are diminishing returns or some low-cooldown CC breaking abilities, I'm not gonna switch to HC. And this is where I completely agree with the OP.
I'm glad there are HC players offering their opinions, as the current state of monster CC is one reason I'm unsure about playing HC characters much at the moment.
It must be infuriating to watch your character be hit by nightmare, frozen, arcane laser, vortex back into a frozen and arcane to death without you ever being in control of your character. In an instance like that, one damage reducing cooldown probably isn't enough, and why would you use two when you could just play a barb and have WotB up all the time?
Removing the CC immunity from WotB is one option, as is adding a long cooldown CC immunity skill to all classes. IMO, adding a CC immunity skill would end up turning the skill bar to 5 slots instead of 6 as it would be too useful for all different styles of play to be worth missing out on. Another option could be to have CC immunity added to lots of different skills, eg. CC immune for 3 sec after using leap/iron skin/war cry (replace with other defensive skills for other classes as necessary).
Other people's suggestions about giving the monster a longer internal cooldown for all abilities, spacing out champ/elite packs better and increasing the cast time between each of their abilities all come back to the same thing. Unavoidable monster abilities are a pain in the ass.