Black Hole and Quicksand, especially in supernova decks. (and obviously Amber Nymph, but that doesn't appear in supernova decks)
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Full ack. Usually, after two or three turns, you can drop the game.
So what would be strategies against? Enchanting / Protecting seems like a good idea. But who has got a bunch of those so you get one for sure at the beginning? Hm...
I play three (yes three!) Protect Artifact in my favourite 60 card rainbow deck, not only for quicksands, but for explosions/steals/pulvy as well. Most arena decks have some reason to make PA useful, as well as probably half the false gods. (and the FGs that you don't need PA for, most of them are the easier ones where you can spare a few dead cards) I also play four Sanctuary in the deck ever since the SoG change.
I'm surprised at the lack of Fire Bolt answers, because it is definitely high on my list.
After a few turns of quanta generation, it can kill most of the commonly used creatures, and with a few more turns it can kill almost any creature. Just one card to kill almost anything is far too powerful for an Element that can support it well.
I don't find fire bolt too bad. It'll kill most creatures, if there's enough quanta available, but most decks I've used don't have an issue with that. (then again, I generally prefer control decks, and firebolt is weaker there) The AI is often pretty bad with it too, doing things like playing two of them on a Firefly Queen and then passing the turn, letting it live. Firebolt can be deadly against the right deck, but there's a lot more answers to it than there are to some of the other cards on the list. High hp creatures, immortality, creatureless decks, discord, burrow, and reflective shields are some of the answers. Something like quicksand, the only solution is PA or a deck that doesn't rely on pillars, like immorush.
Speaking of immorush, I figured there would be a lot more cremations on the list. With the right draw, it can win in 4 turns, even with the newest version producing -1 fire quanta.
I suppose the cards we all hate will depend in large part upon our play styles. Myself, for example, I prefer control decks, so extremely fast cremation rushes are a big problem for me. As I mentioned, I play a rainbow control deck usually, and with four sanctuaries, a card like Discord is almost meaningless against me, but someone playing most mono/duo decks is going to go berserk every time a discord hits the table. Furballdn mentions bonewall as a card he hates, but someone using firefly queen, pharaoh, or epinephrine probably thinks bonewall is easy to overcome. Rootranger mentions fire bolt as a card he hates, suggesting he probably likes decks heavily based on creatures with low hp (6 or less) that it can reliably kill, but somebody using flying titans will laugh at it. And that flying titans player is going to hate reverse time and antimatter, and the rol/hope player will laugh at antimatter, and so on...
The cards that appear the most on this list are going to be the ones that are extremely difficult to counter and/or good against almost all decks. A card like Thunderstorm rapes rol/hope, but it will probably never be mentioned in this thread, because it's nearly useless against most other decks. A card like quicksand or steal, however... well, most decks use pillars or permanents (even if those permanents are just pillars) and there aren't many counters aside from simply not putting out any targets for them.