The encryption is often cited as its primary selling point here, but I don't think that's too representative. Certainly it isn't for me and most other Telegram users I know. I don't care much about that as I don't really trust any messaging app enough to send really confidential information through them.
For me, for example, he selling point is having an interface as slick as Whatsapp's, but not being tied to the phone, not having to necessarily give your phone number, sending whatever attachments you want, and getting a searchable log in the cloud.
Bots are also a popular feature with many users.