Email filtering rules (what is considered spam/bulk) are always closely guarded secrets. If they were not, everyone would be bypassing them and they would instantly become useless. Gmail's "promotions" folder is the new name for the "bulk" folder that they were previously using. I would strongly suspect that sending a large volume of similar messages would be a huge factor in Google's automated email sorting algorithm.
Gmail "bulk senders guidelines" (http://support.google.com/a/bin/answer.py?hl=en&answer=81126) contain the same advice that we give our customers: Use a dedicated IP address, include a DKIM signature (1024-bit or more), and publish SPF records. They do not offer any kind of whitelisting services, each sender being responsible for maintaining their IP address' reputation. The "cleanliness" of your contact list (number of delivery errors) plays a part in the process, but end-user email sorting is mostly done through pattern recognition and behavior analysis of both the sender (frequency, volume, content, etc.) and the recipients (reporting as spam, how much time between your emails being read and being deleted, etc.). The exact mechanics are not publicly accessible.