Lots of stuff.
Your power requirements
Maximum combined 12V load of a potential PSU
Efficiency rating
Potential overclocking of components
Future expansion
Trustworthy OEM
Suitable number/type of PCIe cables
Modular cabling
Dimensions of PSU (some cases have limitations in depth)
Length of warranty
There's also a lot of technical detail, which can get pretty complicated but tends to concern things like voltage ripple.