My personal impression is that Haiku offers a purely desktop centric OS where everything needed to provide a base desktop OS is integrated top-to-bottom. This is unlike Linux desktop distros where there’s lots of difference in the userspace section of what makes up the desktop environment.
Now I actually like both approaches, I like the extreme flexibility Linux offers and I also like the top-to-bottom desktop environment integration which Haiku offers, both have their strengths and weaknesses.
Haiku’s strengths are that like previously mentioned everything which is part of what one would consider a ‘base desktop’ comes straight out of the box and are therefore well integrated rather than separate parts which may or may not fit very well together (which is largely the downside to Linux flexibility).
The biggest problem Haiku has in it’s poor software availability, this in turn is a result of it having a very hard time attracting developers which at this stage is crucial.
Now, one might think that with the huge amount of open source applications out there this would be easy to fix but in reality porting many of these ‘staple’ open source offerings like Firefox/Chrome, LibreOffice, Abiword, Inkscape, Blender, Gimp etc are not trivial, particularly due to the large amount of dependancies which are commonplace on *nix + X platforms but all need to be painstakely ported to Haiku, which is reflected in the available ports which I believe are mainly SDL based games which by comparison are extremely easy to port.
This leads to a shortage of the type of common foss applications people would expect to be available in just about any system and also severely limits Haiku’s useability as a possible day-to-day system.
But there is no simple solution, there is only so much available manpower with the necessary know-how and time to spare and the only way to improve this situation is by getting more developers aboard which again is hard due to the lack of aforementioned application availability.
Thankfully there’s Google Summer of Code which helps lure in new developers but from a project perspective they tend to be more about tackling system needs rather than application needs, which is understandable.
Looking beyond the Google Summer of Code code influx the real boon to Haiku is of course if some of these students find that they want to continue to work on Haiku once their project is complete as has been the case with some of the previous students.
There are other issues like hardware compability but in my opinion the lack of software is the big hurdle when it comes to Haiku having a wider adoption.