you only have to compare its feature set to that of an average phone (let alone a so-called "smartphone" - i still don't believe this category even exists) and you'll see maemo isn't there yet.
even if one day the operating system reaches feature parity with smartphones, the device will still be hampered by the physical aspects of its UI (lack of physical buttons to perform certain operations).

the N900 is a smartphone as much as it is a kitchen radio or a video camera or a computer for word processing.