Pretty sure I've seen these posted here, but my experience with crappy games has usually come from one of these areas;
Most people should know what I mean when I say that a 75% deck at a "cEDH" table, or somebody playing land destruction against low level jank or new players, leads to crap games. But can also extend to a group of more casual players not providing a setting for a more competitive player to use their decks - I believe everyone should have decks at all levels so you can accommodate different playstyles, and a rough level of power should be established before you sit down for a game.
- Players whining excessively
I don't know a better way to phrase this, but when there is endless complaint about one thing or another to the detriment of the mood around the table. That player you can never, ever attack or destroy anything of theirs because its unfair and they are being targeted in their mind. Not people who make their opinion known, as this is healthy for the gaming group, just the ones that seem to find fault with everything.
- Player count or Player interest
Games with too many players, and/or players who are not paying attention. Players who stop in the middle of their turn to talk about something for ages and ages. People who spend ten minutes mathing out each turn when the rest of the playgroup averages a minute or two so except for their biggest turns. No, I don't want everyone to sit down silently for some srs buisiness cardboard, but some basic etiquette so nobody's time is being wasted. Either play cards or don't!
- OTT Rule enforcement - aka "missed triggers"
Some people like to play the game 'properly' and some to play it casually. "End turn - wait, whoops, I forgot to play my land" can lead to different responses at different tables. I'm not faulting people who want to play it that way, but its not for me, and makes games very unenjoyable if someone is trying to enforce tournament style etiquette on my casual cardboard.
I am completely over this terrible format forever. It always leads to four hour games, where most of the planes just seem to boardwipe you, or propel someone ahead, and what you do doesn't seem to matter. I remember one game, where I was the player going last, and the player just before me got a "swap the turn order" plane so I sat there for ages while everyone took two turns. Numberless games where all my stuff is just destroyed pretty much at random, or lol double mana time then move off of the plane while everyone is sitting around twiddling thumbs. I think Planeschase could be fixed but the addition of the planes cards as-is has me at the point where I'll just pack up and go if that stuff comes out.