From the 3.3 patch notes:
Misdirection: Redesigned. Instead of having finite charges, it now begins a 4-second timer when the hunter using Misdirection performs a threat-generating attack, during which all threat generated by the hunter goes to the friendly target. In addition, multiple hunters can now misdirect threat to the same friendly target simultaneously.
This is a sweet, sweet change. Since we hunters put out such AWESOME DPS misdirect is a blessing. Wait, no, Feign Death is a blessing… When I see a fellow hunter cast misdirect before me I shake my fist in frustration (at myself for not getting there first). But now we can all cast it and blow our dps loads up front and not have to worry about getting too much aggro.
The four-second timer should enable us to squeeze out an extra shot or two. Another win for Hunters everywhere!