(credits to poster Strigoì on US forum, i felt need to post it also here)
While the raid damage output of Frost DK is solid, the spec has significant design flaws that make the spec unenjoyable to many.
First off, the spec is once again reliant on Breath of Sindragosa in raid environments. This is not inherently problematic, but the current iteration of this build could be classified as degenerate. Currently, Frost DK has really low damage output outside the Breath window, as such Frost DK essentially revolves around having 70+% uptime on Breath to have reasonably competitive damage. This means that anytime a player loses breath too soon, they feel utterly useless until the next breath window. It is fine to lose damage from making errors but the level of punishment for messing up breath is not commensurate with the reward of executing it super effectively. It also creates a type of play that a large number of players find exceedingly nerve-wracking.
Secondly, Obliteration, the alternative to Breath, is also considered a highly punishing build. A two-handed Obliteration build was theorized to be the best during SL Season 3 but was generally not adopted by top players because it is super reactive and also brutally punished for losing uptime during its Pillar windows. You also have to have excellent reflexes and timing to not overwrite natural KM procs during your Pillar window. It doesn’t help that the Tier set for Frost DKs exacerbates this problem, by having KM not be consumed sometimes, which creates more opportunities to overwrite KM procs during the pillar window.
Third, there are too many dead and awkward talents. Soul Reaper from the general tree is functionally a dead talent for Frost. It does not interface with Frost’s kit at all and even if it did crazy damage would still feel bad to play. Frostscythe is a dead talent, it doesn’t do enough damage to be worth taking and it isn’t affected by the talents that improve Obliterate despite functioning as a replacement for Obliterate in AoE situations. There are 3 talents that affect Remorseless Winter, which feels a bit overkill for an ability that isn’t a fundamental part of the spec’s rotation. Unleashed Frenzy feels really bad at a 6 second duration now that Icy Talons duration was extended to 10 seconds.
Finally, all of the above create a very irritating environment for players. Baseline abilities feel very unimpactful most of the time (Pressing Obliterate outside of KM feels really weak). The most popular build, Breath of Sindragosa, causes players to throw away their battle rez utility during their damage window, which not usually a problem for raids becomes a huge problem in M+. Heck, there isn’t currently a well-defined build for M+ for frost right now. The Tier Set feels more designed for Obliteration but also succeeds in making it more challenging to play. Frost, while performing well in raid, feels very disjointed and awkward at the same time.