In the real world you almost never hear something on one ear but not the other. Even if something is on your left, your right ear still hears it (differences in timing and volume inform your brain on the sound source location). Exception being if it's something really quiet right next to one ear, but that's relatively rare.
So when things are mixed "improperly" (it's subjective), it's very distracting to me. I don't need to force mono everywhere, but it's very common in amateurish channels, and surprisingly also in movies and TV shows. Big productions tend to mix assuming you'll play on speakers (where it's fine to have something playing on just one channel/speaker, since both your ears will hear it), but when it mixes down to stereo and you listen on headphones, it's soooo common for them to pan something 100% to one channel when the source is supposed to be on that side. Like, somebody speaking to the left of the camera, and it comes 100% on the left channel and 0% on the right one. It's so unnatural and annoying to me.
So when things are mixed "improperly" (it's subjective), it's very distracting to me. I don't need to force mono everywhere, but it's very common in amateurish channels, and surprisingly also in movies and TV shows. Big productions tend to mix assuming you'll play on speakers (where it's fine to have something playing on just one channel/speaker, since both your ears will hear it), but when it mixes down to stereo and you listen on headphones, it's soooo common for them to pan something 100% to one channel when the source is supposed to be on that side. Like, somebody speaking to the left of the camera, and it comes 100% on the left channel and 0% on the right one. It's so unnatural and annoying to me.