Whatsapp is a really limited messaging platform. It is a plain extension of the combo SMS/MMS that runs on the internet connection on your smartphone. Here is a list of annoying features of the service:

• You user ID is your phone number. Just as with SMS or MMS, someone can only address you if that someone has your number and it is a Whatsapp user.
• The service can only run on any internet-enabled phone. This is probably the most clear advantage over competitor services: it can handle almost any recent and modern mobile OS. However, it misses key devices such a phone without a phone subscription, tablets (even the ones running the supported OSes) and a classic PC.
• Single association device-user ID. A user ID can only be available on a single device and that device can only handle a single user ID. Related to the previous bullet, one would like to take a lengthy chat to a more comfortable typing device such as a PC.
• Messages lost at a device swap. Although it is possible to backup your message history and put it into a new device, the procedure certainly requires some technical skills, and may not be always possible (forget it if you go from Android to iOS).
• No major differences to legacy messaging services. The service allows sending text, pictures, audio and video clips, your location and a contact card. It supports group messaging, just as MMS. And that’s it. It certainly misses some key services such as voice or video calls. Any decent messaging service nowadays does this by default.
• Does not synchronises your contact information. It’s a terrible failure of the service in my opinion. Especially when you get into a group conversation for which you do not have some of the phone numbers of the participants. You will have to engage in a awkward name request.

Popularity is certainly due to the platform ubiquity: almost anyone with a decent phone can enjoy the service. This is crucial factor especially in developing countries were feature phones are the most common ones.

You may think that mobile operators will be against a competitor of the (sometimes unexplainably expensive) SMS or MMS. However, given the fact that the service can only run on internet-enabled phones, mobile carriers have a nice handle to lure you into data subscriptions. Otherwise, you will only use the service when you are on a Wi-Fi which may be frustrating for you and your contacts.

I personally prefer cloud-based messengers which keep the whole messaging history in the cloud to access it whenever you like. Not only that, you can roam through all the devices you have and move on with the conversation.

The time will tell how the service will evolve under Facebook’s umbrella.