The common thread linking most of these gotchas is scalability. They're things that won't affect you while your database is small. But if one day you want your database not to be small, it pays to think about them in advance. Otherwise they'll came back and bite you later, potentially when it's least convenient. Plus in many cases it's less work to do the right thing from the start, than it is to change a working system to do the right thing later on.
Source: Nine ways to shoot yourself in the foot with PostgreSQL, an article by Phil Booth.