Haswell CPUs add C6 and C7 power phases, which are low amp power phases normally handled by the motherboard's voltage regulator. Most PSUs can steadily supply this (around .05 amps), but some cannot supply low enough amperage, or it varies based on the load on other rails of the PSU, so essentially they will either overload the CPU when it enters that phase, or cut power entirely. I've heard, don't know myself, that in the BIOS you can disable C8 and C7 if you want, with minimal effects.
Long story short, in theory most any PSU will work, but if you're not willing to take what may be a big risk you're probably better off getting one that you know is Haswell compatible. Here is a compatibility list from Tech Report (click here