Main difference between the N900 and previous launches is the complete break in compatibility between versions.

when the N810 came along, it was OS compatible with the N800, and OS 2008 built on that.

I used a 770 for a while (about 6 mths before I got a N800), and there was "hacker" versions of OS2007, and OS2008 produced by Nokia to provide a fair degree of usablity. OS2008 really pushes the hardware of the 770 to the limit.

With Maemo5, Nokia have left it to the community to try and provide backward compatibility. I do not think there is a single N8x0 user who thinks this was a good idea.

applications in Extras still by and large work as they always have done, but development for the N8x0s has stalled.

Should also add, that I reflashed my N810 this weekend, and that everything I had is still in Extras.