1. No it isn't. You can continue to use the "classic" DPS system.
2. You can make separate versions of your content for phones vs. tablets. Note that in the new DPS system all content can be displayed on all devices (even fixed layout content) and we'll scale and letterbox/pillarbox as necessary. This dramatically reduces the amount of versions of your content you have to make. Also note that even with the previous DPS system our guidance is to only create one rendition for tablets, not separate SD and HD versions.
3. Nothing's changed here with the new DPS system. You're in complete control over the structure of your content. If you still want to publish "issues" just put all your articles in a collection and call that "July 2015", for example, on a browse page. Ads are the same as they were in the classic DPS system: you do them as individual articles within the collection, created in InDesign or as HTML.
4. We cache previously downloaded articles for offline viewing automatically. We are working on adding support for explicitly marking a collection for offline viewing, but it isn't currently available in the viewers.
5. DPS 2015 doesn't support filters, which is probably what you were using to manage multiple languages of a magazine in the same app. With DPS 2015 you could do a main browse page that offers language selection, and then after that have all the localized version of the content grouped into collections.
6. The DPS 2015 and DPS Classic systems are separate. If you want to offer your back issues in DPS 2015 you have to use the content migration tool to migrate them over. It's really easy and works fine.
7. Social sharing is available on iOS in DPS 2015, just like in the classic product.
I'm with Helen on this one! I'd really like to know how advertising works in the new Adobe Digital Publishing Solution. I'd love to upgrade to it, however I still want readers to read our publication just like they do currently. Advertising funds the production of the digital magazine and if it's hidden away and not noticed, then this is a very big problem.