When Nokia started to produce the N series of devices they marketed them as "The computer in your pocket".
Some of the N series were outstandingly good (once all the bugs and gripes had been ironed out). Still got my N73 and N95-8GB today.

It all went wrong for the N series starting from the N96. Seems like they never worked on the memory management to accomodate all the extra bells and whistles, to the point that the if you had more than a few widgets on screen at any time the N97 would crash or restart because it ran out of memory.

During this time Nokia were challenged many times about their "Computer in your pocket" marketing statement.
I reckon the N900 was Nokia's answer to all those challenges.
As for the reasons why it "never reached it's full potential" as some put it were numerous and they were not all Nokia's fault. The problems with the N96 and N97 didn't help. The networks were less than happy with the device so they refused to stock it on the grounds that it failed their software testing process (they couldn't install all their corporate junk on it).

I'm surprised how many people think my N900 is running Symbian....or Android even. Most of these people are actually sales staff in mobile phone shops. Shows how out of touch, ill informed and blinded by Apple and Google they are IMO.