At this point you can stop reading if you're a Hunter and just assume the worst. But if you want to see how you're getting nerfed to the ground, read on.
The rationale behind the changes is that Blizzard has been doing a lot of internal testing, along with the beta of WotLK, and has determined the other classes have not been able to keep up with the Hunter DPS output.
The list of nerfs is wide ranging. Volley nerfed by 30%. Steady Shot now gains 10% of Attack Power instead of 20%. Kindred Spirits has been nerfed by 25%. Readiness no longer affects the cooldown of Bestial Wrath.
The list of changes is not exhaustive. Ghostcrawler makes a point to say that these are just the ones they feel are ready for testing. He hopes to see them up on the PTR before they go live.
The full 14 point list of changes after the break.