From a capacity point of view, the +12v amps is the most important metric. That is what the most used components depend on, particularly the graphics cards.
Cheap psu's will advertise high wattage, but it is implemented on other, lower 3.3 and 5.0 voltages which are lightly used.
A quality psu will deliver it's advertised power continuously, not just at peak.
A quality psu will deliver that power at 50c, the temperature inside a case. not at 20c, room temperature.
The best psu's will have higher efficiencies, over 90% for gold rated units 80% for bronze, I think. More efficient units lets the fans turn at lower speeds, making them quieter. Cheaper units may be 70% efficient.
My short list of quality psu's will include Seasonic, Antec, Corsair, xfx, and PC P&C. You can safely buy any modern psu from them.
In the absence of any information, the better psu will weigh more as a rule, but nobody advertises the weight.
Generally the Antec High Current Gamer Series are good.
The HCG-400, HCG-520 and HCG-620 models are made by Seasonic for Antec.
The HCG-750, HCG-750 OC and HCG-900 models are made by Delta Electronics for Antec.
There is also the new Antec High Current Gamer M (semi-Modular) Series with all four models made by Seasonic. Three of the four models haven't appeared on the market yet but they should be arriving soon. So far only the HCG-520M BLUE model is available.
For power supplies that aren't trusted I will always try to find out the OEM for that specific model. If it's made by an OEM that has a reputation for making PSUs that get poor reviews by the reputable PSU review sites then I will recommend that people avoid them.
DC output quality is extremely important. Poor DC output quality will manifest itself in unwanted and very difficult to diagnose system behavior. It will also degrade/kill the capacitors on your motherboard and graphics card and even within the PSU itself over time.