Luke Wroblewski nailed this long ago - "Mobile Signup Forms must Die" http://www.lukew.com/ff/entry.asp?1678 - it's not rocket science but if you allow your old school web thinking to define your app UX, you will fail. If you have any clue about mobile audiences you use gradual engagement to allow them to get on board incrementally.
The average mobile user probably has about 30 seconds to give you until the next push notification from WhatsApp, Snapchat, Instagram, Twitter or whatever distracts them and their gone. If you waste that time on forcing them through a signup process, don't expect to see uptake.
And failing by login isn't a new thing for Yahoo! For many happy Flickr users (myself included) they killed it for us by forcing Yahoo login onto the site. From http://gizmodo.com/5910223/how-yahoo-killed-flickr-and-lost-...
> Yahoo's RegID solution turned out to be a nightmare for the existing community. You could no longer use your existing Flickr login to get to your photos, you had to use a Yahoo one. If you did not already have a Yahoo account, you had to create one. And you did not even log in on Flickr's home page, upon arriving, you were immediately kicked over to a Yahoo login screen.
...but that's another story. I'm still bitter