I personally don't think DKs are broken in any way... But, to me it does feel like Blizzard are unsure as to what direction to take us.
I wanted to throw a few ideas out there and see what others would like to be done to revamp the DK class somewhat before the next expansion. I'm not just talking about DPS or survivability either :-P Hell, I'm bored so this will likely be pretty long.
Disease Changes.
Diseases are a fairly large part of the DK arsenal but, IMO they could be re-worked/simplified so that each spec only has one disease.
Blood - Blood Plague, applied by blood strike or heart strike - Causes base nature damage over time and reduces the target's attack/casting speed by 15%
Frost - Frost Fever, applied by icy touch or howling blast - Causes frost damage over time + adds a magical vulnerability effect
Unholy - Ebon plague applied by plague strike - Causes shadow damage over time + adds a physical vulnerability effect
The damage done by each disease would obviously be more than a single disease would do today to compensate for the lack of the other one. Blood would lose access to both plague strike and icy touch. Frost would lose access to plague strike and unholy would lose access to icy touch. (I'll address the dispel/chilblains issues later on!). This change would go some way to improving the desirability of the mastery stat for both frost and unholy.
Making Crit more Desirable
Killing Machine devalues crit, which is a shame. To make crit more attractive KM and Sudden doom could work like chaos bolt. They could still proc off auto attack swings, or even auto attack crits but could have extra damage based on your current crit %
Obliterate
To make mastery more desirable for 2H frost Obliterate could be changed so that, if frost fever is on the target obliterate does 20% extra damage as frost damage (less than the 25% extra it does today to compensate for the armour penetrating effect of frost damage + Mastery). Similar to the shadow damage element of scourge strike.
Icy Touch + Howling Blast
Icy Touch's damage buffed to be the go-to single target Rime ability OR single target rune ability for DW. Howling blast's single target damage reduced below that of Icy Touch. HB's single target damage and cleave target damage now the same. This should mean use of HB in AOE/cleave situations and IT in single target situations.
Scourge Strike
Put simply the bonus strength from unholy might could be reduced or removed all together if blizzard would just buff the damage of scourge strike. It could have a higher modifier from weapon damage and deal 50% extra damage as shadow to compensate for the loss of frost fever = Better burst, possibly slightly less AOE ability depending on mastery value... And, for the love of god the shadow portion should be able to crit!
Ghoul Changes
Huddle is replaced by fortifying roar when not empowered, reducing ghoul damage taken by 50% for 10 seconds. When empowered the ability acts as a reality shift, putting the empowered ghoul in a shadow realm and making him invulnerable to any movement imparing effects, damage effects and abilties that lose control of the Ghoul for 10 seconds.
Dark transformation only requires 3 stacks of shadow infusion. Shadow infusion increases damage and HP of the ghoul by 15% for every stack.
Raise dead cooldown reduced to 1min for unholy.
Unholy now replaces Control Undead with Dominate Undead. Dominated undead minions are considered permanent pets. An unholy Death Knight can keep up to 3 different types of undead minion and summon them from a pet selection bar which spans from the Raise Dead ability. Different types of Undead minions have different abilities, the 4 main types being:
Casters (e.g Banshees - ranged spells, silence or HoTs)
Light DPS (e.g. Gheists - quick melee abilities, stuns or Bleeds)
Heavy DPS (e.g. Skeletal Swordsman - slow melee abilities, roots or bleeds)
Tanks (e.g. Abominations - Slow Melee abilities, absorbs or hook grips)
Soul Reaper
Shadowfrost damage, cannot be dispelled, still does damage if the DK dies.
Grasp
Chains of ice could be replaced by an ability called grasp (mainly for cosmetic reasons), the DK plunges his weapon into the ground and a death grip-type hand appears underneath the target, snaring him. Grasp can also be glyphed to dispel. Can be used while moving, would look awesome :-D
Talent Changes
Unholy Blight - Same as today but applies all 3 diseases (would normally only have access to one of them)
Plague Leech - 45 second cooldown, explodes for 100% of the remaining disease damage on a target, consumes the disease.
Rolling Blood - Blood Boil applies your spec's specific disease as well as buffing the damage component of Blood Boil slightly.
Lichborne - 10 second duration. When Lichborne is activated you're granted 100% RP in addition to your undead presence. Once the effect expires any remaining RP is removed.
AMZ - Now baseline for all DKs. Spell graphical effect unchanged, lasts 10 seconds, any friendly team member inside AMZ takes 35% less magical damage, replaced in talent tree by:
Mark of Blood - 1.5 min cooldown, place a Mark of Blood on an enemy. Whenever the marked enemy deals damage to a target, that target is healed for 5% of its maximum health. Lasts 15 seconds.
Purgatory - Upon taking a killing blow you're protected from death for 3 seconds. When the 3 seconds are up you will take damage equal to the damage taken during the previous 3 seconds over 10 seconds as a DoT.
Death's Advance - mainly unchanged, breaks roots/snares on use
chilblains name replaced with "sickly retreat" - victims of your diseases are slowed by 50%, Grasp roots the target for 3 seconds.
-------------------
Yeah, went a bit OTT. Would love to see some sort of defile ability too but that's not likely!
I know a lot of the above is pretty unrealistic but meh, any other ideas? :-D