What made them interesting wasn't the hardware, though the full keyboard on the N900 was useful but the fact that it was Linux phone based on Debian complete with the ability to apt-get install whatever the hell you wanted. Including compilers, developer tools, openssh, and whatever you could think of.
The big difference between the N800 and the N900 was that the N800 was more of a tablet form factor (released years before the ipad or the iphone were a thing). The N900 was smaller and had a proper phone built in. It could connect to mobile networks and make phone calls. The N800 was by operator decree basically crippled to be wifi only. Operators absolutely hated the notion of an open platform like Linux running on devices connected to their networks.
The victory Steve Jobs imposed on operators was basically getting them to beg on their knees if he would please allow iphones on their networks. He completely turned the tables on them. The first iphones were exclusive to some networks only and people cancelled their subscriptions if they were on the wrong network. That's why iMessage is a thing and SMS texts are a thing that is no longer generating meaningful amounts of revenue for operators. There was no off switch for iMessage. Steve Jobs basically told operators to take it or leave it. Likewise MMS was not a thing on the first iphone. Nokia mistook that as a fatal omission. A missing feature. The truth was that MMS was dead as a doornail the moment 3G internet connectivity became a thing. Why have operators act as a middle man? Steve Jobs had no patience for any of that.
Anyway, Nokia still obeyed the operators and it ended up crippling anything with software potential. There were big discussions about having Skype on these things. The N800 had that. And a webcam. You could make video calls with it. In 2007. The N900 did not have Skype. And it was positioned as a developer phone. Worse, it had to compete internally with Symbian and the Symbian team was in control of the company.
So, it was positioned as a developer phone and the N9 was launched (2011) similarly crippled in the same week that they shut down the entire team working on the OS. That was around the time Symbian lost out to Windows Phone and the beginning of the implosion of Nokia's phone division.
The key point here is that Nokia had an Android competitor long before Google launched the Nexus. Before they had the Nexus, they were dual booting N800s into Android. I flashed mine with a development build at some point even. Nokia screwed up the huge chance they had there long before the iphone was about to launch. The N770 launched late 2005. The iphone wasn't even announced until 2007.
Nokia did not understand what it had and crippled the platform instead. And then it dropped out of the market completely.