You have to realize the timeframe that N900 was released: 2009. Back then, it was way ahead of most other smartphones (including iPhone), and had multitasking to boot. The resolution was touted as "better than DVD". It had a commandline out of the box, and not some watered down, misplace all the system files version either. It was basically Debian on a phone, with smooth, beautiful graphics, a decent package management system, and tons of possibilities (Rovio ported Angry Birds to it!). Oh, and did I mention it also had Qt, including Python QT, with IDE options (including emacs) on the phone? Sure, the N9 was pretty, but it had no hardware keyboard, and the N950 doesn't count as you still can't easily get one (you had to be an active Maemo developer to get one).

I bought a N900 and it may have been ahead of the iPhone in tech specs but it was way behind in the feel of day-to-day usage. True multitasking could drain the N900's battery in an hour even if the phone was idle. Because the stylus was included many apps required it, so a lot of time was wasted unholstering the stylus. The resistive single-touch screen doesn't support gestures. Maemo didn't support portrait orientation for one-handed usage.

Hand up - still use one as my only phone. It definitely has its flaws, but the ones you mention have really never been an issue for me, either due to fixes, updates or workarounds.

I somehow get 1.5-2 days use out of the thing, per charge, despite frequent multitasking (browser, SMS, notes, ssh etc.) and typically running it with a slight overclocking to 805mhz. A script switches on airplane mode while I'm asleep, so that probably helps.

Others have mentioned the screen, but I'm really comfortable reading on it. The Opera browser makes the peck and hunt game on desktop versions of sites obsolete. All the fuss about resistive/capacitive differences is beyond me. It's really quite easy to move (things) about on the screen, and I'm not into splitting hairs.

Some have mentioned the Psion series in this thread. I have to agree, the 5MX was a brilliant design for a mobile device. If that keyboard was coupled with modern, open HW in a similar phone it would be game over. For me at least.

I have been considering buying a folding bluetooth keyboard to type up notes and posts on the phone, but I think that's pushing it - my main 'creation' work still happens on the PC, for better or worse.

I don't do a lot on it that you can't do on any other phone, but it does give me a warm feeling that it's at least open and not phoning home at the end of the day to update Sauron.