The N900 was my favorite phone by far. Most of the reason come as a programmer, not all users might value them.
First of all ot was a true Linux phone. Running a derivative of Debian, complete with apt etc. Most of the apps on it, even the app to make a phone call where preexisting open source Linux applications with a nice mobile UI. An UI that was clean and consistent without the need off branding that other platforms suffer from.
I could simply write bash script for it to extend its functionality. For example I remember writing a script that would record and sent recordings of phone calls to my server.
I installed Pidgin the chat client and used it to chat with my friends when MSN was still popular in my county. It truly felt like having a PC in your pocket.
The keyboard was the best keyboard ever made in my opinion. I could actually type blind on it sensing a message without looking at the phone at all. I still own 3 N900's with the hope of every managing the find the enthusiasm to rig in the another phone or a pi.