Here are my list of the worse class card from each class and why:
Demon Hunter: Fel Summoner - Demon hunter has some insane cards this expansion, but this is by far one the weakness. It has pretty aggressive stats, but a rather expensive card which has low impacted when played and can be easily ignored if your opponent wants to push face damage. Also if Demon hunters want to cheat out a big Demon, Raging Felscreamer is just alot better and consistent to use instead. so overall made the top place as the weakest of the Demon hunter cards.
Druid :Ysiel Windsinger - In wild this card has some Potential with really big spells like Infestation, however in standard at this current point, there is no spells worth running it. It just too expensive with no real combo pieces to make it a good card.
Hunter : Nagrand Slam - I just think this card is really cool, but also pretty hard to play or set up outside cheating it out with Kael'thas Sunstrider. If you wanted to play a hunter deck, a dragon or aggressive one is mostly easier to play. The no Duplicate decks could win without a 10 mana card which may end up not dealing the damage it needs to. Overall, just too expensive in a class who does not require an expensive spell to win.
Mage :Imprisoned Observer [/card]- This was a coin flip between [card]Imprisoned Observer and Starscryer as both don't see play, however I think Starscryer is more consistent,, while Imprisoned Observer is much slower and can be played around by your opponent. It was played alot at first, but has dropped out of most deck lists as there are just better things to do on turn 3 and by turn 5 when it becomes active, your opponent has better minions to play and you might be catching up tempo if you did nothing on turn 3.
Paladin :Lady Liadrin - Don't get me wrong, this card is not bad and does see play, but out of all the Paladin cards, i think its probs the worse as it requires alot of set up and has rather mediocre stats. If you dont play enough Librams and other spells, It just ends up sitting in your hand for alot of the game waiting to be played, which makes it pretty meh at most times.
Priest :Imprisoned Homunculus - Overall for a Class card, it just is pretty weak. A 1 mana 2/5 with taunt is pretty good, but a 3 mana 2/5 with taunt is very average. Compare it toImprisoned Vilefiendwhich is a neutral card which comes into play turn 4 as a 3/5 with rush for 4 mana which is pretty good. Just not strong enough as a class imprisoned Demon, should be at least a 3/5 with Taunt I think.
Rogue : Cursed Vagrant - A very expensive card, even if its pretty ok for a class common. For a rouge to play this on turn 7 instead of doing something like Galakrond just does not make sense. It is in a class which has better late game options and Rogue is not really running Stealth Packages to make this card good.
Shaman :Vivid Spores- I dont think Shaman has enough small creatures that they want to re-summon. Unlike druid which wants to always be on the board in an aggro deck, Shaman rather push damage as much as possible and spending 4 mana to re-summon small creatures just seems too expensive. In a control deck it is just too slow, so it does not fit into any of the current decks in standard.
Warlock:Keli'dan the Breaker - There is only 1 effect to make it worth it in standard to play this card and that is Plot Twist. Drawing it early on, later when you are on the board or at a point where you dont need it, makes the card a worse Flik Skyshiv. I think it needs alot more support to make this card anywhere near good enough to run in tier 3 or up decks, which i dont see happening for a long time.
Warrior :Bulwark of Azzinoth - A card which is just not worth running against any classes in the current Meta. Mage, Demon hunter and Rogue can easily chip the durability down and even if you can re-quip it, i just think its too gimmicky in a control deck. You rather play good turn 3 or 4 weapons over running a legendary card which requires you to play against a class with no weapon removal, no way to chip damage or small minions.
I think vivid spores is a good card overall, but it doesn't fit a lot in the meta. I was trying it out and seemed to work pretty well with rush minions, like galakrond or the 4/4 faceless, helps you clean some HUGE boards.
Besides galakrond, I can hardly see it being useful because shaman doesn't rely a lot on big minions or deathrattle ones, so...
Also, talking about Liadrin... I'm so disappointed at that card, I got it and was so happy to try out pally (I have Tirion and can't use it because of meta), but after some games and trying different things... I can't see its value, it's hard to get a huge Liadrin and I would've needed a control deck to play around it, meanwhile most control decks nowadays can out-value me by far. I don't have all the cards I'd like tho.
As for other cards, I think some will find it's place in the meta, for example warrior's bulwark. Today I faced a control warrior that was using it and it made my midrange deck unable to kill him. Give people some time to discover new things!
I've played a bit with Imprisoned Observer. I'm not saying it's the greatest card, but I think what it does is stops the opponent (DH looking at you) building a board for turn 5, not the effect on the turn it actually goes off. You can pretty much go into 6 mana with an empty board state.
I don’t agree with imprisoned observer being the worst card for mage, it is at least used in some highlander mage lists and it is quite useful against demon hunters. I think apexis smuggler is just worst, at least if we are talking about standard. There is no secret mage list doing well on ladder and even if there was I am not sure it would include this card. about Keli’dan, I don’t think it is the worst card for warlock. I have tried it and it doesn’t feel so bad to play even without plot twist. There is actually a Galakrond Warlock list that uses it without plot twist. I think Shadow Council is simply a worst a card right now, it is not included in any list and it has a really low winrate.
Keli'dan? Are you drunk? This card seems very good even without its condition met. Siphon Soul is ran in Warlock decks, and Keli'dan is basically Siphon Soul trading the 3 Health (0 mana) for a 3/3 (2.5 mana). Before the nerfs, there were two very well performing Galakrond Warlock lists and one was running Keli'dan.
Disagree on Nagrand Slam, it has a natural home in Highlander Hunter, very versatile card and has won me many games. It's a "board-in-a-card" that doubles up as possible removal for small/medium boards AND a 6 to 12 damage reach/finisher, which also allows you to play Dinotamer more freely without fear that you'll run out of reach if properly responded. Coming after a DQ Alex turn it also means that an opponent (like priest for example) has likely spent some resources dealing with a huge board already, and might not have an answer for another one (and yet another one if you're counting Zixor Prime). Being able to play 0-cost dragons from Alex on the Nagrand turn is also pretty good.
If you want to pick Hunter cards that are underperforming and have seen little to no play, you should look at Helboar and Beastmaster Leoroxx.
I also think Vivid Spores isn't as terrible as you make it out to be, I often pick it in Totem Shaman from the spell lackey. I think the problem is atm Shaman is so weak that can't afford to run yet another card that requires you to be ahead on board otherwise is a dead draw - Totemic Reflection and Splitting Axe already are that way... but in a deck whose win condition is basically to make small minions/totems stick to the board to buff them and possibly finish with Bloodlust, a card like Vivid Spores would be basically a win condition. Soul of the Forest does the same in Druid to threaten Savage Roar and is played right now, the difference is that Druid cheat mana like nothing and can afford it, Shaman can't... yet. I wouldn't be surprised if Vivid Spores becomes viable when more tools are added in the next expansions.
OK. Now go for the polite disagreements : cards that are not currently played but may be in the future.
Warlock:Keli'dan the Breaker - a 6 mana hard removal that gives you a body is a decent control tool, and it may find its place in a future contorl warlock. Door to darkness or whatever its english name sounds like a much shittier candidate, as it is not reliable enough for a combo deck and not good for anything else.
Mage :Imprisoned Observer- Due to extreme diversity of mage decks, most mage cards are good this one packs some punch. He may be a very solid card if some midrange mage deck shows up. On the other hand, I see the Solarian Prime as an opeless outgunned legend.
And now the "are you serious" disagreements
Paladin :Lady Liadrin - She looks awfully weak, but it turns out that getting a hand refilled with 0 cost +1/+1 buff librams is actually a powerfull move. She is one of the win conditions in Pure Paladin
Priest :Imprisoned Homunculus - A fantastic Turn 1 play for control decks, or a great way to throw one mana. Great defensive card, played in some priest control decks.
Mostly agree with your post, but Imprisoned Felmaw is seeing a lot of play actually in Highlander and Boar combo decks. I don't like the card and I mostly don't play it in my Hunter decks, but sometimes it's a pain to deal with when played against me.
I haven't seen a single Beastmaster Leoroxx instead. People are mostly trying to combo it with copied/buffed King Krushes, but the exact same plan with Boars seem faster and more effective, so...
Kelidan the Breaker is a really solid card. You would play it in ANY control deck. Its 6 Mana hard clear on a 3/3 body. That is just plain good.
The Hunter card has also overall good results being used. I don't think it's that bad.
The Priest card is also decent
And for you to say a legendary card is bad because it requires you too play a certain deck is just above my head. Liadrin is a fine card and makes Libram Paly possible alone. You could make the same argument on Hanar if you don't have secrets in your hand.
You hit on some of them, but half you might just have pulled out of a hat.