Product itself and ability to expand/incorporate new features based on the client requirement. Support Staff is awesome and response time is very fast. Always available for working session on the questions, issues, account setup and new feature onboarding.

The real-time syncing of recipients is so much more advanced than having to upload a list and watch it get out of sync as it uploads.

Analytics and event management - Also one of the most powerful type of message i've ever used in my professional career is the Conversation. I like the variety of options, the fullness of the interconnectivity, and the options for your outbound campaigns according to the many different marketing channels. It works well internationally, so it's flexible per client based on the type of English you need to use (and I'm certain other languages will prove it's flexibility as well).