OOC Chat
The biggest difference in the actual gameplay is that the ideas of "standard action" and "move action" and "full-round action" are gone. You get three "actions" per round. You can do whatever you want with them: move three times, attack three times, cast a spell and fire your crossbow (most spells are 2 actions), move attack and retreat, whatever. The only real limitation is that if you attack several times in one round, the later attacks take a "multiple attack penalty", which is analogous to the old iterative attacks.
There are many numerical differences but nothing else anywhere near as major in the system itself, and nothing that will really surprise you too much.