Unholy DK main since they were put into the game in WOTLK as I came to the game in late TBC.
I have played on and off since, every expansion (with MOP being the least played) until live TWW.
I have almost completely stopped playing unholy DK in latest expansions and I couldn’t put my finger on why I just didn’t find it fun / interesting and have switched to rogue/warlock main since, playing it in MOP has made me realise why in a lot of ways.
The rune system in MOP is perfect, you actually have to think about what abilities you are hitting and plan ahead for converting death runes. Compared to live where runes as a resource are no different from energy effectively…
Disease spreading is easy and part of your rotation (blood boil + pestilence glyph).
Soul reaper is a fun “dk only” mechanic, trying to perfectly get a mobs HP to 35% for when it ticks to maximise soul reaper hits is fun in dungeons and open world and gives some satisfying big crits and the bonus of haste even if you don’t time it perfectly.
Bring back DK presences, adds further depth to the class and makes unholy playable right at the start with the free added haste.
They gutted most of these things on live and DK no longer feels like a unique class because of this imo.
EDIT: Adding to this now i am 90, still finding it really fun with the interaction between soul reaper and necrotic strike. Diseases actually hitting hard feels like true unholy DK class design as well with it being the “melee warlock”.
Having more fun on MOP currently than I have had on live for a while, good job to the dev team behind pvp changes for MOP.