1. They live forever.
Most people know that big parrots are a 50+ year commitment, but even cockatiels can make it well into their 20's. Budgies (parakeets) are a 10+ year commitment, and a finch can be around for 8 or 9 years. Changing families can be very hard on social birds, so people considering a parakeet, lovebird, cockatiel, conure, or parrotlet should seriously consider their plans for the next couple of decades.
2. Birds are evil.
Parrots are jealous little beasts and have a tendency to fall madly in love with one person and hate all potential rivals for affection/attention. Even cute, cuddly baby parrots will eventually hit puberty and develop a passionate hatred for your spouse, your children, your parents, your other pets, your friends, or anyone else who visits your home. Not every bird does turn into a feathered machine of pain and anger, but any parrot can suddenly decide to terrorize the people you love. Alternately, your bird may fall madly in love with your spouse/child/other family member and suddenly decide they hate you.
3. Birds are messy.
Smaller birds tend to be the worst mess-makers, but even big birds love to fling food and stuff everywhere. It's a major form of bird entertainment. Even through careful engineering I've only ever managed to direct the mess rather than contain it. Daily vacuuming is pretty much unavoidable, if birdseed falls in damp places it tends to put down roots and sprout, and you will still be finding petrified crud years after you cease to own birds.
4. Birds will tear up your stuff.
In the wild parrots spend pretty much all day shredding things in search of food and nesting materials. In captivity this desire for massive destruction cannot go unquenched. Even provided with an infinite quantity of shreddable-toys intended for shredding, parrots will occasionally decide to sample your library, data cables, or something else that you love. This is usually accompanied by innocent and adorable behavior intended to throw you off your guard.
Do not be fooled. Birds are monsters. Birds are descended from carnivorous dinosaurs, and they have not forgotten this. Birds will destroy your home, your marriage, your family, and all of your possessions. Do not let them into your house. Say no to birds.