There are a lot of huge announcements here. But in particular, I'm excited by the Assistants API. It abstracts away so many of the routine boilerplate parts of developing applications on the platform.
Apart from RAG which many others are discussing elsewhere in the thread, a big one is gradually summarizing long conversations that exceed the context window. This had to be done manually before when using the api but it sounds like it's built in to the new assistants api.