It became more customizable in terms of skinning, the component rendering now is a different concept (you are suggested to have each component as a two-complements, the logic and the representation), so that representation may be changed w/o affecting the logic. It will also enable in certain situations a virtual layout, where some components will be able to share the canvas so to reduce the number of display objects actually created. However, the features side overweight the optimization side, so, overall the components became bigger in size, as well as the framework itself grew in size significantly. Most important change regarding the size and visualization is that new components use TLF for text rendering and UIComponents for skinning. The states have been given more attention and this has also reflected on MXML language in that now it has special syntax for addressing states. Flex CSS acquired namespaces, although it still seems like work in the progress. Flex compiler now can compile "native" graphics from textual format - GFX, although a nice feature, still requires some tuning until it gets really useful.
Well, this is what I could remember at the moment.
