Its should go with out saying but lets get it out of the way anyway. The PSU should be good/reliable and deliver clean power.
Then I think things can be divided up into two categories. No frills PSU's and enthusiast PSU's.
Basically the the "no frills" line should be as cheap as possible (without compromising power delivery quality). Does not mater if its not modular, loud as hell, ugly, not sleeved cables. Just make it good and cheap. Something suitable for super low budget build's, folders ect. you get where I'm going.
Then for the enthusiast PSU wish list.
Make it silent. Not necessarily fanless but think Ty-140 running @ 700rpm silent.
Modularity is good. Even more awesome would be if there was extra cables so you could either have more sata or molex connectors. And be generous with the PCI-E cables. For example a 850W unit should have 6x PC-E cables don't you think (enough for 3 high end cards)?
Still pretty basic stuff but here are some more out there ideas.
I think it was mentioned all ready but some debug led's would be nice. Even something as basic as a little led above the modular connectors that show if power is being drawn form that particular cable.
For cooling It would be nice to see some high power (like seriously high power +1kw units with optional water blocks). I know there has been some PSU's with WC support like the Silentmaxx Fanless 500 but I have never seen a high power one.
And now for the weirdest idea of all. I wan't a PSU that you can reverse the air flow off meaning it would suck in air instead of exhaust it giving you more flexibility in how air is going to flow in your system potentially also cutting down on the PSU noise as it would get fresher air (would be very nice for people with most of there other stuff WC'd). Also combine that with a dust filter so it does not suck in ... well dust.
It might also sound weird but I think there would be a market for optional un-traditional cables. Like cables that are single strand instead of multi-strand so you would do 90° bends and have the cables actually hold the form or cables where the insulator is clear showing off the copper wire inside. That would be pretty cool.
Oh and someone should make different angled PCI-E adapters so it would be easier to route the cables.
Something in the style of this-->