A downside of Google's shared storage system is that it affects both Gmail and Google+ Photos, not just Google Drive. Gmail used to offer 10GB of free storage, Picasa Web/Google+ Photos only 1GB and Google Drive - 5GB. Small photos (< 2048x2048) and short videos (less than 15 minutes) uploaded using Google+ Photos, as well as the documents, spreadsheets and presentations created using Google's Drive apps don't count towards your storage limit.
"If you exceed your quota limit, you'll receive warnings in each product and you'll need to correct the issue as soon as you can. Otherwise, you'll be unable to upload additional items to your Drive or photos to Google+, and, after a period of time, incoming messages to your Gmail account will be returned to the sender and you won't be able to send new messages," explains Google.
Now that Yahoo Mail offers 1TB of free storage and Outlook.com "includes email storage that expands to provide you with as much storage space as you need", Gmail's 15GB limit doesn't look that impressive. Maybe Google wants to encourage people to use Google Drive for uploading files, instead of using Gmail attachments.
Yahoo's Flickr service offers 1TB of free photo storage. "No limited pixels, no cramped formats, no memories that fall flat." Suddenly, Google's photo offering is less impressive: you get unlimited photo storage, but only if you resize the photos.
It looks like Google no longer has the edge when it comes to free storage. Gmail offered 1GB of free storage when its main competitors only included a few megabytes of storage. Now roles are reversed.