The killer feature of any messaging app is always the fact that your friends are using it, or that they are willing to start. While this factor is different for everyone, having apps available for many platforms is a good start. Telegram is used a lot by the AlternativeTo team and it is available on almost any major platform, including Android, iOS, Windows Phone, Linux, Windows and Mac OS X.
We really like that Telegram is open (API), free (forever) and heavily focused on privacy and security. It is also feature-rich without feeling overwhelming. Your chats sync to all your devices, if you want them to, and the support for group chats is very powerful.