Well, for a start there's plenty of WONTFIX bugs and wanted features in the closed components on the N900. If they were opened, some of us potential developers (I include myself) would have been (a) motivated at fixing those, to "scratch our own itches", (b) as a consequence develop more familiarity with the platform and so tip the balance further towards choosing it for serious developing, and (c) maybe even make the platform better.

It's a big decision, which platform(s) to develop for, because it takes so much time to learn how to do really interesting things well on each of them, and we are not at the stage were smartphone apps are easily cross-platform. Qt and Web Runtime are nice UI sugar. Useful, interesting, commercially sensible, but only part of what makes the most interesting apps and mods. The other parts are under the hood somewhere, and the APIs available are not enough, especially for interesting mods, bugfixes and performance enhancements.