New Resource Pack screen:

How it works, with top of the list taking priority:

Also, serverside Resource/Texture Packs are reimplemented.
[1] For those of you that don't know, this is when the server serves out Resource Packs instead of the users being required to download them individually. The max size for server side Resource Packs has also been increased to 50MB.
[2]Jeb has written an article on upcoming changes to biomes and the like:
https://mojang.com/2013/08/minecraft-world-generator-update/My personal highlight is that biomes now have classes, so it is very unlikely that clashing biomes will generate adjacent to each other. (Desert next to snow biomes, etc.) Of course with this type of generation, it's impossible to prevent it 100% without risking oddities, but it will be uncommon.