So I’ll start off this post by saying that the game developers at Blizzard must be masochists. I wonder what stuff we could add to seriously annoy the player base?
I’ll go first: rogues or now druids (oh joy) that make you draw all your cards, or the priests who use your deck against you by copying cards from your hand or deck.
Don’t discount the possibility they’re just dumb.
those are fun decks of course they added them for us to have fun with
copying is really fun i like adding some neutral cards to make then synergize with my opponent ones if a deck is popular enough
sometime i just run some to know what type of deck my opponent is running!
like the spell to discover a card from my opponent deck or drefdge and change cost to 6 !
i do it with druid too a jungle giant deck with azalina so i can make her cost 0 and copy my opponent hand !
I wish stealing decks would run into more clueless decks that are horrible to steal 
it’s a tossup for me between that, secrets decks, and OP quests. If i had to choose i might go quests.
This Rouge made a 30/30 Edwine Vandercleef on turn 1 with all these coins and draws and prepares and Snowflipper penguins and stuff, I was dead on turn 2
Thief Priest Mirror is peak Hearthstone, absolutely the most fun you could possibly have playing the game.
A deck can be annoying because everybody is playing it.
A deck can be annoying because It ruins your deck.
A deck can be annoying because it delays the whole game and does not really have a win condition.
Having said that I have not seen any annoying decks since the last patch.
Have they deleted the milling decks?
No, but it’s/was not good.
Atm there are a ton of Shudder Shaman since the Seed nerf.
Maybe Mill Druid become playable again now 
It’s good when it’s against paper, but bad when it’s against rock