You can tinker with it. You can install different operating systems. You can interact with it via different peripherals (keyboard, stylus). Current devices are at odds with this idea. Closed and with programmed obsolescence baked in.
I really think there's room for a platform like Maemo/N900 today. Especially in a post-Snowden world, where privacy has become a major concern. Neither iOS nor Android are good enough. Many privacy conscious users have moved to CyanogenMod, which is OK but a bit immature. It's an Android patch after all.
If there was some stock phone with average specs that you could buy, like a Raspberry Pi, then I'd bet we'd see many more options in the same vein as the n900.
I'm using an LG Cosmos 3 [1] and I'd kill even for access to modify the BrewMP OS and re-flash it. That's all I'm asking for! Even that seems impossible.
Many projects, like Neo900 (or even Jolla Tablet) have had loads of trouble trying to manufacture something that is affordable.
It all ended up pretty badly for Nokia, mostly due to suicidal management decisions first by sticking exclusively to Symbian well beyond its use by date, then by switching to Windows. Impossible to understand when they had a winner product in-house.
The Maemo era ended with the N9, outstanding software and hardware killed before it had a chance to succeed [1].
Jolla was a Maemo spinoff, but without the Nokia hardware expertise and financial muscle it was quite hard to succeed. Still, I hope they manage to put things together. I think the world needs a mobile platform with a better and more open userspace than what Android and iOS offer.
Other projects like Pyra [2] and Neo900 [3] are trying to accomplish the same.
[1] http://www.engadget.com/2011/10/22/nokia-n9-review/
[2] https://boards.openpandora.org/pyramain/main/
[3] http://neo900.org/