I've been following the Haiku project for a bit and I find it quite nice to bring back to life the old BeOS. It was without a doubt a good OS back in the days and I'm sure that if it was luckier and with some more advertisement it would be really bid today.
However there are features that are lacking really bad (and this is my point of view, so please, don't see this as an attack to the project).
In my view Haiku misses one big issue to make it an OS to the people, it misses a localization kit. Something that will format currencies, numbers, text alignment, dates and the whole texts on the base applications to the language and locale of the user. I think this would break the wall of non english speakers to use Haiku.