I can literally -- and I mean that word in the proper usage -- think of
nothing in the N900 (or earlier models) that changed the world in any way. And I still own, and occasionally use, my N800.
Prior to the iPhone the acme of portable touchscreen interface design was the Newton Messagepad 2100. Its interface was superior to Maemo in practically every way, despite being discontinued ten years prior.
Maemo feels like a crappy, half-assed attempt to implement some of the Newton's magic on top of X11, and with weird desktop mouse-oriented design elements (menus, scroll bars, combo boxes, Windows-style tree-views, even cursors in some apps) bolted on awkwardly without any thought about their appropriateness to a tablet operating system. I wrote an extensive article comparing the two way back when, when I was doing a moderate number of contributions to the N800 platform. Maemo was amateurish at best, with an incredible array of inconsistent and obtuse iconography, several inconsistent sizes and designs for the same exact kind of GUI element (text buttons, icons, ), a complete lack of state persistence, absolutely horrible fonts, and a desperate need to be backward compatible with X11, hardly the best choice for a mobile OS GUI foundation. It was clearly built by linux software developers and not GUI designers.
iOS didn't borrow anything from the N900: it borrowed directly from the Newton and from MacOS and NeXTSTEP, when it wasn't inventing things from whole cloth.
Thankfully now Android has shown us that Linux can shine in the mobile pad market, if you jettison X, stupid archaic design elements from desktop GUIs, and design a consistent and organized interface from the ground up considering the needs of the device and and user.