Widespread familiarity - most employees are familiar with the gmail interface and have used drive in some capacity. Drive facilitates great document collaboration and has revision and version tracking built in. Google Groups is a distribution list on steroids - allowing a converstaion history to be saved within the group and not just individual mailboxes so new teammates can have historical conversation context.
Room for Improvement:
Integration with traditional e-mail clients (outlook) is not there. Huge learning curve and wasted time trying to make calendar sync seamless for end users. Google chat is nice for a quick IM, but not nearly as robust as a Lync or Skype (and thus many people use personal Skype accounts for business conversations, calls and screen shares).