Its simple: The mechanic of spell pushback overlaps too much with spell cancelling in PVP. Spellcasts in melee are overcountered. Disagree? My rogue can keep a warlock spamming the fear button from completeing a single fear for over a minute and a half with a combination of slice and dice + melee haste, the standard cheap shot/kidney shot/blind/CloS/gouge and of course kick. It is clear something is wrong here.
A warrior spamming hamstring and his swings on my mage can easily make a frostbolt cast 6 seconds long, even casts that were 2.4 out of 2.5 seconds from completing before he entered melee range. Furthermore, he can then cancel it 5.9 seconds in (and lock the frost tree out as well of course).
I'd do away with spell pushback in PVP altogether personally, but I can see why an adjustment to the pushback times would be more controllable.
I propose:
-Spell pushback has been changed. All hits currently capable of causing pushback in PVP must now also deal at least 3% of your total health in a damage in order to succeed in slowing your spell.
-A spell can only be pushed back a maximum of half its casting time (I.E. maximum casting time of an improved fireball is now 4.5 seconds, improved shadowbolt 3.75 seconds, etc.) In addition, each successful pushback now removes 0.25 seconds from a casting bar.
I have a rogue, a mage, a warrior, and a warlock, so I'm on both sides of this issue. Any non-QQ discussion is encouraged, and I'll make sure to respond to anything well written.