As a windwalker, I have a boatload of ways to counter a frost mage. But last night playing against a mage/priest in twos with my resto druid partner, I still eventually came up against a deep freeze I couldn't avoid and it killed me. My partner was PoM-sheeped, and had trinketed a Dominate Mind earlier.
I can Nimble Brew one.
I can trinket a second (if I dont use trinket saving my partner).
I can Diffuse Magic just before deep for a third sometimes.
I can even Paralyse the mage as his Deep Freeze goes off.
That's far more ways to avoid Deep Freezes consistently than most classes have, but eventually there is going to be one where you have just run out of outs, and your partner is CC'd or unable to stop it as well. I'm not saying Mages are too good even against WW 1v1 because I certainly have the advantage. I'm saying that when you get caught in a single unmitigated Deep Freeze as any class, you will die 100 to 0, and that is the problem.
It could be just that burst is too high in general for all classes, but you can't really argue that historically Mages have not garnered a high amount of protest from the PvP community in really every season. They have always been a strong class, especially against certain comps. Sometimes factors just push them over the line into overpowered (like original frost bomb), and Blizzard reins them in slightly. Deep freeze is only so strong because of the 100% crit chance and the fact that they have CC on demand for a second target with Polymorph, Counterspell, or now Frost-Jaw