4. Low-Level Flight
This one is a no-brainer. Altitude is like money in the bank. If anything goes wrong, a pilot flying near the ground has less time to assess and deal with the situation, and the chance of a mistake by the pilot resulting in catastrophic consequences is greatly increased. On top of that, many pilots flying at low level are not content to merely cruise along in straight and level flight. Instead they are often involved in showing off to their friends by buzzing their homes and doing what is essentially low-level aerobatics.
Low-level flight is the only airborne deviation that does not involve a mistake by the pilot, but is a conscious act. It is legal to fly at any altitude above the water or over a sparsely populated area as long as you stay at least 500 feet away from any person, vessel, vehicle or structure, and an emergency landing is possible without undue hazard to persons or property on the surface. However, this means that all you have to do is fly within 500 feet of one person to be reported for a violation, so it just is not worth the legal and physical risk.