I hope you're trolling Sessho, otherwise...lol.
1) You are able to pew pew a shaman caster but then wonder wtf you are almost dead when you ignore a MUCH weaker version of the shaman caster just sitting over there being completely ignored? It wouldn't even matter that its a boomkin, it could have been any caster and you chose to ignore it.
2) DKs are designed as an anti-caster class. It is almost entirely what they do best. You have:
Anti-Magic Shell - 75% magic absorb and you gain runic power. Cannot be purged, stolen or dispelled (even if druids could do any of those things).
Gnaw - pet stun
Strangulate - silence
Mind Freeze - school silence and interrupt
Death Grip - the best gap closer in the game
Chains of Ice - slow, applies disease, gain runic power. Druids cannot dispel magic.
The last two abilities can basically be spammed on rotation.
I am not even going to talk about Hungering Cold or Anti Magic Zone.
Druids have:
Moonfire - lol dot. Dispellable. No chance against Blood or Death Coil+Lichborne or Death Strike healing. Silenceable.
Insect Swarm - another lol dot. Dispellable. No chance against Blood or Death Coil+Lichborne or Death Strike healing. Silenceable.
Typhoon - knockback. Death Grip >> Typhoon. Silenceable.
Starfire - effing 3.5s base cast time! Interruptible. LoSable. Silenceable.
Wrath - less damage than Starfire, effing 2s cast time, slow travel time. Interruptible. LoSable. Silenceable.
Treants - temporary (30s) little pets which do only white damage. Almost unusable in pvp because of pathing bugs. If you're really really "unlucky" you may get dazed (50% movement speed) a whole 39% of the time their tiny branches beat on you, assuming the Moonkin has specced into Brambles and assuming they even get to you in time. Silenceable.
Entangling Roots - caster druid's only viable CC. 1.5s base cast time. Without Brambles talent, high chance to break on damage. Dispellable. Interruptible. Silenceable.
Starfall - "Shapeshifting into an animal form or mounting cancels the effect. Any effect which causes you to lose control of your character will suppress the starfall effect". Have any friend can basically guarantee that this will never get cast before the druid is dead regardless of your own gear and competence. Silenceable. Controlable.
And for protection against all this incoming pain what do Moonkins get? :
Moonkin armor - like all armor useless against diseases and magic damage, a substantial source of DK damage compared to other melees especially in pvp!
Barkskin - a whole 20% base damage reduction with 1 minute cooldown! Unusable under Strangulate.
Healing - access to all druid healing spells except the aoe heal and incredible Swiftmend cooldown. Requires a GCD and significant mana cost to switch forms. Healing Per Mana is ridiculously low in caster form. If a Moonkin is having to heal themselves they are probably almost already dead and out of mana.
Basically, the chance a competent DK loses 1v1 against an equivalent-geared competent Moonkin is low to ridiculously low (easily less than 20%). I was killing frikkin' level 80 chickens when I was level 74-75 in bgs, ffs! They are the dk's bread and butter. If you cannot kill a chicken as an anti-caster class, good luck facing any other caster and probably consider re-rolling!
Oh and arena is about teamwork...I think you must have realised this by now.