The directory structure was changed so the app folder is now outside of the www folder. Everything in www/build will be auto generated from the app folder. The conference app is a good project to view when you aren't sure if your project is correct. It has a section in the README explaining the file structure. Please keep in mind this section is manually updated and can be out of date at any time, but we try to keep it updated.
The app directory should be your working directory now, and any assets (images, json files, gifs, etc) that your app needs to use should go in the www directory since this is directly copied over to the platform folder. Here is the issue where we discussed changing the directory structure:
If you are having problems with ionic serve make sure to update the Ionic CLI to the latest version needed for Ionic 2 (at the time of writing this is 2.0.0-beta.15) by running: sudo npm install -g ionic@beta
Let me know if you're having issues with ionic serve on the latest CLI.