If your ass is not making it to the session on time at 2 pm like we agreed on multiple times in a row, I will not trust you to be on time for an event.
If you are making absurd demands of the DM and try to bully them to include your overpowered item (including homebrewing it for you and then you throw a fit when you don't like the results), I will not trust you with real power. etc etc
EDIT: I am not looking for DMing advice here, I already do sessions 0, read articles and have dmed and played a lot. I am mostly noting how small shit can tell you if you can rely on/trust somebody for more serious stuff and that TTGs have a lot of small shit coming together at once.
Oh yeah. I mean, there was one guy we had in our dnd group who wanted to like do horrible things to every npc. Like I won't talk about it further because it was honestly disgusting. We never invited him back.
Had a guy like that who had played a lot of DnD in a group of people still trying to figure everything out and made everyone really uncomfortable.
That's one of many reasons I do a session zero to lay down the law. There are things i will not do or tolerate as a dm and i make it clear at session zero that agreeing to and adhereing to those restrictions is something I ask the players to respect while I'm pputting in the effort dming for them.
A very good idea, especially for new groups or ones with more than one new player. Often if there's just one new player, they fit in or drop out with the rest on their own.
It really helps to get everyone in the same room and talk about what kind of campaign they want to do. Silly? Serious? Lots of politics? Hack and slash dungeon crawls? High magic? Low magic?
Then everyone can starton the same page, knowing what they're getting in to. Experienced and consientious players will be able to shape their characters and goals to fit in with thegroup's expectations, then build from that.
My gaming experience improved so much once I learned about the session zero concept.
I wholeheartedly agree.
It also works as a first line of defense; some especially toxic
types get mad at the concept of a session zero. I'm not kidding; I speak from personal experience. The source of the rage is apparently from their suspicion that the game will be a "safe space/hugbox" and them getting mad about even the idea of people consenting to content helps screen them out that early on. 
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"It's all just pretend and those aren't real people anyway!" excuse enjoyers seem to have quite a bit of overlap with others that hypothesize about how living breathing people are actually NPCs/simulated/not-really-conscious
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