The interface may seem to be improved but even then, it cannot be the same for the Ribbon UI or perhaps the standard toolbar as enlarging it may be a difficult way. Sure, Office client version has the Touch Mode to expand the spacing that makes things suitable for the touch screen devices but unfortunately, certain of those tablet devices are making the buttons look too small. I don’t know what the latest version of Office mobile looks like on Windows Phone but I assume that the buttons can’t be this small.
Interface-wise, there are newer features for Office Online apps. There will be features that will make those apps catch up with the client counterparts and the Metro counterparts. While OneNote itself is free across multi-platforms, the others are not.