My bad experiences have mostly been covered by others, but I'll reiterate them:
1)
Uneven playing field. When one deck clearly gets to dominant lead, and is a vastly different powerlevel than the rest. This doesn't mean that every game that ends up in Archenemy is bad - sometimes one deck will naturally pull ahead after a series of plays and boardwipes, and ride that value train to victory. What I'm talking about is when the game is clearly lopsided from the get-go, with one deck clearly constantly a step ahead or above other decks.
My playgroup mostly avoids this as we're a small local group, who have played with each other for years. We know our playstyles and powerlevels, and react accordingly. Sometimes when we reveal our Commanders for a game, someone will realize they've pulled something out that's clearly above the level, and put it back. I've retired several decks that had ended up getting a bit over-tuned for my group.
2)
Anti-climactic ending. Specifically those that have no (or stupidly little) interaction. Sure, I get it, someone's got to win. But when we're having a back-and-forth battle for several turns in the late game, with the power dynamic shifting every turn as everyone tries to squeeze the most out of their diminishing resources... I don't want to see a combo or spell that just says "Got a counterspell or I win?" I'm here to play with a variety of cool Commanders that I can try to build spiffy strategies around weird abilities. I don't want that game to say that I need to run blue, or everything might suddenly end. That's boring.
I get that combos are a part of magic. Heck, I even run a few. But I just don't like the combos (or big spells) that simply just win from the hand, with limited interaction for 99% of magic.
3)
Spite and Hate. It's a game folks. It's not a personal vendetta or attack against your character. You don't need to storm out in a huff and fury because someone attacked you with a solemn, or because you think they should have disenchanted something else. Maybe they should have, maybe not. You don't have their knowledge of their hand and/or strategy, and your viewpoint and assessment
will be different. That doesn't mean they made the wrong play. Have your vengeance in game... but please, leave it at the table. It's not personal.
4)
Not being able to play the game. One of my first memories of a truly terrible game was fairly close to the beginning of my Commander career... nearly a decade ago, and I still remember the game, and the feeling. I was playing a
Tibor and Lumia deck (which eventially became a fairly tuned storm deck... but this wasn't it yet). The deck was built with a simple idea - "Doing stuff is fun. I'm going to do stuff." It was filled with card draw, cantrips, and cheap instants, so I could do a large
quantity of things, even if most of those things was useless.
Then I got hit by an
Identity Crisis, immediately after a full board wipe. I had lands, sure, but the loss of my full board, hand, and graveyard was quite destructive. To be fair, it was a fantastic and valid play from the person playing it, but from where I sat, my deck that was supposed to do a lot of stuff, had nothing. I literally just played topdeck mode for 8 more turns, dying to incidental token damage to gain combat triggers. It was mindbogglingly dull and dreary to simply sit there and do nothing for only god knows how long.
While
Identity Crisis isn't a broken card by any stretch, that memory of just being bored and having no impact remains one of my worst experiences. Today, I tend to build a lot of resilience across my board, hand, graveyard, and even lands now, to try and mitigate any amount of that kind of blowout, but nothing is perfect. I also advocate strongly against cards that lead to or create those kinds of situations, which is why I have been adamantly against Iona for the longest time.