The ability to do group chats, voice calls in real time with video also available is great! It can be expensive (especially if you are not using it enough).
I would like more integration between groups of people that have different apps installed so we could use them all at once instead having only one option per app or chat service. We had no issue when our phone numbers changed over from landline phones to cellphones but this didn't apply here since everyone uses smartphones already anyway. Using WhatsApp was easy because there were many other users who used WhatsApp as well which helped make things easier.