This is one of the most important hurdles left for Ubuntu Touch
The main contention of Android users who wish to try the new Ubuntu Touch operating system is not the main apps, but what they expect to find. Sure enough, Ubuntu Touch is a complete OS, but it also needs a replacement for WhatsApp and a messenger that supports the XMPP protocol (at least).Ubuntu developers managed to implemented a lot of new apps in Ubuntu Touch, but one is being missed most by its users.
Almost a year ago, a developer announced his intention of bringing the much coveted WhatsApp functionality to Ubuntu Touch, but nothing has been heard since.
A wiki page for Ubuntu has been created specifically with this aim, to add support for XMPP (and possibly other protocols) to the messaging app.
It's unclear whether WhatsApp support is doable by a third-party developer or if the direct implication of the WhatsApp developers is needed, but I feel this is one of the last major hurdles stopping the widespread adoption of the operating system from Canonical.