What I like to look for in anything I buy is quality when budget affords. I want a PSU that is both quiet and reliable, using top quality components, not this cheap POS that is overdue for replacement. To me, the name Seasonic means quality (at a price).
I look for a good review on jonnyguru.com. I don't know anything about electricity so I leave him to find any potential problems or concerns with the power supply construction. Other than that, moar moar moar efficiency. More expensive is fine IF the power savings from running the PSU 24/7 at $0.20/kwh will cover the price increase.
In for quiet, clean, reliable power on a single rail with lots of amps. Modularity is always a bonus, with cables of sufficient length and flexibility to make routing and clean cases easy.
Of course, the first is the power I need to get the job done, that's a given. The others are above. I could care less about noise at this time. My PC needs to be stable and fast. It's pretty quiet, otherwise.
I prefer fully modular power supply so that I can use only those cables that I need, also more then 6 SATA power connectors, as hdd and odd are now all SATA, its kind of a requirement.
I look primarily for quality - so the power supply won't blow up the rest of my system. (Currently I'm still running a PC Power & Cooling 510-PFC from the early 2000s.)