Newly implemented (13w36a and 36b) Resource Pack list and picker is not refreshing after you copy new .zip file to resource pack folder. In order to check new resource pack one has to quit and restart Minecraft.

In previous versions it was possible just to copy resource pack zip files to appropriate folder and they became visible in resource pack list ingame. Lacking this feature is a bit annoying for resource (texture) pack makers by forcing them to restart minecraft every time they want to test updates to the pack they made.