Netherwind vs Enigma.
Stereotypical mage with full BWL loot:
http://ctprofiles.net/2553307
For all intents and purposes let's say they're running 30 Arcane/21 Frost (Frost specs without Arcane Instability will do a little less damage) and no outside factors such as Winter's Chill, Brilliant Wizard Oil, etc.
Assume a +5% damage bonus adjustment for 8 piece Netherwind.
Flat (noncrit) DPS: 458.13
Crit-factored DPS: 523.27
Same mage, just swap out the relevant 5 pieces for Enigma.
http://ctprofiles.net/2553385
Flat (noncrit) DPS: 444.13
Crit-factored DPS: 511.05
However, that's somewhat foolish - let's swap in the C'Thun and exalted Bronze Dragonflight rings (since it's no guarantee your raid will ever see Ritssyn's), and the best of the other available loot from the zone in the other slots.
http://ctprofiles.net/2553615
Flat (noncrit) DPS: 461.91
Crit-factored DPS: 534.73
There are some big intangibles, though: streakiness of the Netherwind proc, +hit, crit rate, mob resists and mlvl. A fully enchanted Netherwind mage is only going to have +5% hit from equipment and no more. The Enigma mage will have +7% hit from equipment and will get more on demand as he needs it; the maxed-out Enigma mage will have +9% hit from equipment, and more as he needs it.
Let's assume the Netherwind mage gets no Netherwind procs in a given fight:
Flat (noncrit) DPS: 436.31
Crit-factored DPS: 498.35
Also, if Netherwind procs less than 10% of the time in a long fight, he will inevitably do less DPS than his Enigma-geared counterpart since the net DPS gain will be lower than 5%.
Another thing to take into consideration is that Netherwind procs eat mana faster (unless of course you happen to get Clearcasting at the same time). If you're forced to burn mana timers faster, you hasten its emptying, and once you're dry you're going to be wasting time regenning mana and losing DPS.
The third intangible is crit rate. Critical strike can be pretty streaky and some people just have better luck with landing them than others. That factor is entirely out of anyone's control.
Mob resists: if the mob has frost resist the person with the Enigma set is going to have the advantage. With just some quick calculations the bearer of Enigma will land around 12% more spells on a frost resistant mob than one without Enigma.
Final factor: mob level. There are persistent rumors that Kel is level 64. If that ends up true in empirical testing that means his to-hit is 72% (disregarding any frost resist/immunity he may have). Again, due to the stacking +%hit buff the mage with Enigma is going to have a decided advantage.
I completely understand why people are reluctant to give up Netherwind. The "I win" buff is extremely alluring and hard to ignore. However the math simply shows that with proper gearing the Enigma mage will over time outdamage a Netherwind mage given similar gear.
Take this with a grain of salt if you must. I do not have 8 piece Netherwind and have never been able to do any testing with it. I probably will not still have it until months down the line from now (if ever) due to being locked out of the last 3 pieces I don't have. Also, fuck the drakes and their lousy ass glove dropping system.