Karen reports neighbor for having an "ugly" doormat, neighbor reports her for illegally subletting her apartment: 'She has not spoken to me since.'

If she doesn't want to walk by her neighbor's doormat every day, she shouldn't live in an apartment building. 

There are a lot of downsides to living in an apartment building. The worst part about it is that you can't control who lives in the apartments below, above, and next to you. There's a reasons why NYC co-ops require you to go through an extensive application process; if you're buying property in an apartment building, you want to guarantee that you're not going to be living next to obnoxious, antisocial people. A majority of people who live in apartments don't buy, but rent, so they at least have a little more freedom to move out after a year if they live in an apartment building with irritating people. Even still, it's undesirable to have to tolerate bad neighbors for a whole year. 

Everyone has different standards for what qualifies as a "bad neighbor." I turn my nose up at the residents of my apartment building that never bring their mail in, but I think that's just a me thing. Nobody can be a perfect neighbor all the time, so if you can be forgiving and empathetic to your neighbors when they "break the rules," they will probably do the same for you. I've been lucky to never have a neighbor that would dare report my house parties to the landlord, even though it's forbidden in our lease. It's kind of a "You scratch my back, I'll scratch yours" situation. I'll pretend you don't have a forbidden cat, and you'll pretend I don't have forbidden parties!