400 rating points is
not a lot, even with the stats squished.
Itemlvl 630, Heroic dungeon blue 2H, 118 haste rating
http://wod.wowhead.com/item=110046&bonus=524
Itemlvl 685, Mythic T17 raid epic 2H, 230 haste rating
http://wod.wowhead.com/item=113953&bonus=567
So in 6.0 as a new 100, a single heroic dungeon weapon is 1/4 of that cap. And a mythic raid drop from the very first tier is half of it. Remember these numbers are increased by 10% from attunement, too. This is of course only a single slot. We'll hit 400 haste rating in leveling greens.
At that point Haste is devalued, retaining the high wait times I hate so much. Remember all those parses at 40% wait time in 660 gear are with every piece of haste gear and haste enchants we can get. But we'll be actively trying to
avoid haste. Crit is already very low value due to Killing Machine. Mastery remains very low value for 2H frost due to Obliterate dealing physical damage. And thus the subspec is left with two valuable stats; Multistrike and Versatility.
But wait, Multistrike is very inexpensive in rating per %, so it will diminish quickly in later tiers. And then we're left with just Versatility. The end result will be a migration to DW frost, because it makes much better use of Mastery on gear. This is what happens when stats aren't balanced.
The devs hate the word, but this actually
is a classic scaling issue.