Offline queueing is a fantastic feature! I know someone who lives in a remote area whose phone only gets network connectivity when they walk up a specific hill. In many ways, they use WhatsApp the way most people used email in the dialup days: read and compose offline, then go online once a day to send and receive new messages.

Edit: I sometimes get a chuckle when they say "Look at this cool picture I took", and then receive the picture some days later when the conversation thread has moved on. I guess they'd have stayed in thr coverage area for long enough for the entire image to upload.