Ad 3: I love my moribund N900 for:
0. true multitasking, it's me who decides
1. good hw kb (can't imagine my life w/ virtual kb only)
2. installing &updating apps with apt-get; terminal (it's Linux!), decent shell where you can run your own script.
3. the possibility to tinker with it, experiment, simply have fun
3.5. Fully manual internet connection!
4. awesome photos
5. resistive screen
6. fm transmitter, IR (yay!) - not that useful in everyday life but sometimes.;
7. software: OMP I know no better MP with lyrics on any other system generally averagely better quality apps than in Harmattan.

In decreasing importance.

Because of 0 and 1, Android, iOS, WP are no way

What I miss on N900: lack of some apps, that the system core is outdated, bugs in apps such as camera-ui2, performance (but performance is not that bad0 - you can cope with it)