But first this is not new. Symbian and WinCE/mobile are also similar to the desktop paradigm. You can write your own applications for them. You can buy applications for them. The vendors do not control those applications. (Interestingly, neither WinCE/mobile nor Symbian succeeded in creating a decent market for applications. In a fraction of the time, the iPhone did.)
Second, I am not really sure that the N900 will be as open as you think. Dr. Ari Jaaksi had puzzling comments on the "necessities for open source developers to embrace paradigms necessary to the phone industry like drm and closed source".
Furthermore, you have been intoxicated by the PC/windows/linux model. Everyone here seem to ignore that a smartphone may be a computing platform, but it is not a pc. The pc model emerged in the 90s, before the Internet was popular, and the technological choices which were made at the time reflect that (and we all know what disastrous consequences some of those choices had). A smartphone, with limited battery and computing ressources, always on connectivity, high hardware variability, high bug resilience and designed for a market of non-specialists implies different choices. If you want to make some money, that is.