Packages on debian ARMEL are based off of the soft floating point ABI, whereas harmattan uses the hard floating point one, so packages from Debian aren't 1:1 compatible with ones from the N9.

(note, Debian sid DOES contain hardfp pacakges, but the architecture name there is armhf, and they contain a lot of dependencies that are outdated on Harmattan, since sid is bleeding edge)

All that said, even if they were the same floating point ABI, it still isn't recommended for any deb based distro to install debs from another distro.