the presence of hardware keys on a touchsreen phone. The presence of a "back" button where it should have all been controlled within the confines of a screen. Symbian^3, Maemo and iOS are great examples of this.
In terms of web browsing, there isn't much of a difference on both handsets. The N900 still has some advantages in terms of the full desktop web experience. Flash compatibility and page rendering are two of them.