The poster goes on to suggest that rankings be based on the class, rather than overall Arena population. This means that the percentile used to determine end-of-season rewards will be applied on a per class basis, thereby granting all classes conceptually equal representation. Ghostcrawler gives the suggestion some merit, even going so far as to say Blizzard isn't "above iteration on the design" of the Arena system as evidenced by their proactive adjustments to it.
Ghostcrawler mentions that their primary concern is how players would feel under such a system, as players who play an underrepresented class would then get the advantage under these circumstances. In essence, he fears that "grading on a curve" can potentially award lower rated players undeserved rewards simply based on the class they play. The bigger concern that I see that shifting towards this system takes the responsibility off the developers to perfect class balance.
Let's face it, the classes will never be truly balanced. But that doesn't mean Blizzard should be content with one or two classes completely dominating the environment. They've shown that they're committed to toning classes down if needed (even if it sometimes takes a long time -- sometimes an entire season too long), and constantly issue hotfixes and patches that address this imbalance. Shifting to a class-biased rating system takes much of that responsibility away and promotes complacency not just on the part of Blizzard but on players, reinforcing the idea that some classes are simply better than others and discouraging true effort.
I find it important to point out that Hunters have traditionally suffered low representation from Season 1 through 4, something many players -- including myself -- blamed on the Arena format itself. The Hunter class simply didn't lend itself well to close quarter environments. Blizzard proved me wrong, however, and made several adjustments that allowed Hunters to compete in the Arena format. The result? Season 5 saw Hunter dominance to the point where Blizzard found it necessary to tone the class down.
My point is that even though the suggestion makes sense on paper, it will create all sorts of problems for the game. It's not just class representation that needs to be focused on but even specs. Blizzard has long stated their intention to make virtually all specs PvP (or Arena) viable. Does that merit a spec-biased rating system, then? No, of course not. Striving for class balance under the current Arena system -- the friendliest and most accessible it's ever been -- is what's key, not a retuning of the system to cater to underrepresented classes.
Composition is an even more significant issue than mere class balance, which can be looked at in a vacuum. Blind was made to share diminishing returns with Fear effects to tone down the dominance of Priest (Psychic Scream) and Rogue compositions, while opening up Cyclone and Blind to work together again. These little things work towards promoting composition balance, and is an encouraging sign that Blizzard is paying attention. Working towards class balance is the best scenario here, not developing a system that concedes to the imbalance.