I don't know ONE application that I am really missing on the n900.
Yes, I'm missing some features.
But features can be added in a debian-like system like maemo.

Features can't be added on a closed application on android, iOS or Windows Mobile.
(...And if you want a useful application you have to pay!!!)

Linux has a big tradition. The applications are adapted to the needs of the users. Debian works on hundreds of different hardware systems. It works the same on a big mainframe like on the little n900. Imagine!!!
The programs are adapted at the most different uses.
The program on the n900 and the linux PC is THE SAME (not only similar!). (We don't speak about the optification.)
You can take the output of the n900 application and continue on the same document on the linux pc. And even if the application on n900 has an upgrade the same application on the linux pc has the same upgrade.
The old linux programs are venerable utilities that have be proven by tradition.

Android, iOS and Windows Mobile are only for mobile devices. If you want a usage that is a little bit different from the intended one you are completely lost. Those applications are idiots only for one usage.

Linux applications are universal. They work on every hardware you compile it. They have been tested on hundreds of hardwares. They are useful in different circumstances, with different configurations for different purposes.

The Linux on the n900 is identical with the linux on a big PC.

That's a very big difference to all other OS'es that are only for mobile devices!