I should have included the bit before that. I know about that, but how does that affect its openness?
Aegis prevents messing with some of the system files, but it's not too hard to circumverent/disable Aegis alltogether. Install open kernel or use Inception.
Though I haven't bothered with either. Aegis hasn't (yet) come into my way when porting software to the phone or installing stuff from community repository with apt-get. Much of stuff in community repos are just stock debian armel packages, with slight modifications in control files.
If you know how to compile programs in Linux, then that's the only thing you need to know to port stuff to Harmattan. Install Scratchbox to your computer, log in to it, download sources, apt-get necessary -dev packages, ./configure && make and scp resulting binaries to your phone. For extra points you can ofcourse roll it into debian package and kindly ask at #harmattan IRC channel for your package to be added to the community repository so that everyone can install it on their phones with apt-get.
N9 is just awesome. Swipe UI wipes the floor with Android and IOS + it really feels like a true Linux computer. Elop has made sure it's hard to get, but IMO it's easily worth all the money you throw for it.