Serious post. We can talk about pollution, overcrowding, etc. but here's one problem you probably don't attribute to overpopulation: degeneracy, which, predictably, increases along with population. Why the sudden erosion over the last 70 years of standards i.e. behaving like a civilized human and being non-degenerate.
The larger the population gets, A) the larger the raw number of people who cannot or will not conform to social norms and B) the less cohesive the population as a whole gets. It is easy for 50 people to have common culture; easy for 2,000; easy for 100,000 or perhaps even a million. You cannot have a shared culture with 350 MILLION people. It's impossible. And what happens when common culture disappears? A race to the bottom. Standards everywhere evaporate, the decent segments of society get corrupted by the lesser ones.
Look at many "conservative" men today. They have visible tattoos, unkempt beards, use profanity in everyday conversation, wear t-shirts with juvenile imagery, and eat like pigs. Behavior that would have appalled any normal person at the edge of living memory.
Overpopulation is the root of most of our troubles.