OK, let's limit it to end-user installable applications through the system's own package management which act as the native applications look.

So, on Maemo if you want to include a port it has to be Hildonised, power mgmt aware, installable from Extras and have a UI suited to the form factor of a Maemo 5 lead device.

...but it is only €5, so I'm not too worried about getting the terms exact ;-)