There won't be a killer app for Wayland specifically. It's infrastructure. It's also not the place where all the showy (but ultimately annoying) user interface fluff will be found. It's infrastructure.
The question is what can it do that X can't, and there are plenty of examples mentioned in this thread (including no tearing, high refresh rates, multiple monitors with different DPI, etc).
But the issue is those things are nice to have, whereas X is baked into existing workflows that either do not work on wayland, or only work on a subset of compositors (due to philosophical disagreements), and many of these workflows have been around for a while, and there is no desire (or funding, or time) to change them.
The question is what can it do that X can't, and there are plenty of examples mentioned in this thread (including no tearing, high refresh rates, multiple monitors with different DPI, etc).