Normal difficulty bosses Shannox, Beth'tilac, and Rhyolith took 15% decreases to health and damage, whereas all of the other bosses in Firelands took a 25% hit. Certain trash packs have had their proverbial bite removed, and Alysrazor's tornadoes will now move more slowly. All adds in these encounters have had their health pools and damage decreased in the same proportions to their corresponding bosses'.
On heroic difficulty, all bosses' health pools and damage, along with adds' corresponding health and damage, have been reduced by a blanket 15%, which is a huge nerf for heroic modes. Alysrazor also drops more feathers on both difficulty settings, and her Wings of Flame buff is now a static 30 seconds.
While many players will be upset with these nerfs, I can only imagine the many raid groups that will happily charge forward to finally defeat the encounters that they have been banging their heads against for the better part of the patch. Blizzard has said that there will be more nerfs coming, as this is just the first round, so make use of this time to get as many of the bosses as you can while the getting is hot.