Can't answer for parent, but my personal biggest gripes are:
1) No graphical acceleration yet
2) No application sandboxing mechanism (yet?)
3) A package manager that doesn't seem to have a reason to exist.
To expand on 3: Native Haiku software really has no need for a package manager because it has no need for dependency management. It appears the only real reason to have it is so Qt can be installed as a dependency for the ports that need it. Why not just provide Qt in the base system? According to the developer I talked to, its because they want to encourage people to write against the native API. Ok, so why support this use case at all? Because ported software is better than no software. Either I am still misunderstanding something or this is an incredibly strange decision.
Which isn't to say I don't like Haiku, indeed it understand Desktop as a usecase far better than any Linux distro that ever existed, I just have those particular gripes.
As far as reasons others might not like it:
1) I hear that porting some POSIX software is a bit jank
As always, AnIdiotOnTheNet has valid comments, but you cannot dismiss a journey because its too far, you need to start with the first steps and eventually you'll get there.
1) re: graphical acceleration, it will come when the user base increases, the 3 big players will eventually want skin in the game
2) re: sandbox mechanism, yes it's missing, however since Haiku has a read only package system, the damage a rogue app can cause is much smaller eliminating the need for sandboxing in the first place. Also, the Haiku system is easy to restore, and from a user point of view, I care more about preserving my data, not protecting a system that I can easily restore. This is also a fault of most OS's out there, they protect you from installing a driver, and do nothing for protecting your family photos.
3) see 2
1) re: posix, Haiku has similiar issues as the BSD's in that POSIX != Linux
Regarding your reply for 2: I completely agree, however the immutable packages do nothing to protect my personal files. Any modern Desktop OS should recognize the need to distrust applications by default.
That said, Haiku isn't really any worse in this regard than currently existing desktop OS.s
Personally, I found it painfully mouse-heavy (ditto Plan 9); on *nix, I can use i3+tmux+vimium+keynav and basically never touch the mouse. This matters particularly on laptops where the keyboard is good and the mouse, such as it is, sucks.
