Earlier this month, the FAA's Micro Unmanned Aircraft Systems Aviation Rulemaking Committee (ARC) issued its recommendations in a final report: http://www.faa.gov/uas/publications/media/Micro-UAS-ARC-FINAL-Report.pdf
The recommendations include classifying UAVs (drones) into four categories, based on the risk that they pose to people underneath them. If the UAV fails, it will crash and could cause a serious injury.
If the mass of the UAV is less than or equal to 250 grams, then it would be in Category 1, which would have no additional restrictions (beyond those already in place).
UAVs more likely to cause a serious injury would face more restrictions. For instance, Category 2 UAVs "must maintain minimum set-off distances of 20 feet above people’s heads, or 10 feet laterally away from people, and may not operate so close to people as to create an undue hazard to those people." Category 3 UAVs would not be allowed to fly over crowds or dense concentrations of people. A Category 4 UAV, on the other hand, could do that if it complied with a documented, risk mitigation plan.
There are many interesting details about the ARC, its risk attitude, how the ARC developed its recommendations, and other factors. In particular, the ARC did not consider the likelihood of a UAV failure or the likelihood that it would hit someone if it failed; it considered only the distribution of the consequence (the chance of a serious injury) if it hit someone: "Specifically, the ARC recommends that a small UAS be permitted to conduct limited operations over people ... if that UAS presents a 30% or lower chance of causing [a serious] injury upon impact with a person."
