1) rets offhealing is miles stronger then ench - they get way better heal then ench free of cost every 3d judgement (and since most are stacking haste cd on it is damn low) that they can follow easilly with WoG,
2) their teammate propection is superior to ench is every way since 1 clutch BoP can easilly save your healer from inevitable death, BoS is insane too serving as both a dmg decrease for trained target or out of cc card for a healer
3) BoF is extremly strong too for both healer propection and DK mobility since ret himself can cleanse movement imparing effects of himself (and that opens DK to picking chillblains serving as permaslow since asphyxiate isn't that needed with rets own short cd stun)
4) purge isn't really needed since dk pretty much spam BB and IT providing constant dispell and decease spread whille doing better dmg against higher armor classes then using festering strike
5) ench is 1 time burst per arena match class whille ret can easilly blow cds right away since they'll be up shortly after
6) rets burst is insane, fully geared it's not unseen off droping 300k in 3 gcd
7) enchs generally will have stronger dmg reduction then rets come next patch, but... against plently of teams bubble is a 100% lifesaver (and if you've priest healer then there's still lots of stuff to fall back on like PS and VS.
8) Generally combination of good dk and ret and priest can setup insane cc chains with dark simed cc, stun, fears, aoe disorient. With setup preassure potential i easilly see half games ending in first dark simed cc followed by hoj into silence.
Imo for ench preferable partner would be rogues with their strong lockdown, together they'll be able to peel from priest better and will be caster cleaves bane, as for dk/priest i don't see a better addition then ret. But one thing for sure - MW don't fit this setup as good as priest, not even close.