I'm not going to defend everything xenadu02 said, but I think there were some points that resonated with me even though I agree they could be expressed more constructively.
> Why does ls do sorting? Why does grep do -R recursive searching? How is that "Do one thing and do it well"?
I think these are valid examples of how Unix itself fails to follow the "Unix philosophy" of "Do One Thing and Do It Well".
> The fork/exec model is not "garbage". It is actually a fairly nice alternative to the "provide one API to start a child process and give it a large number of parameters for all possible situations". And you can call plenty of APIs between fork and exec in the child safely, just like from signal handlers.
fork-exec complicates the implementation of threads (see atfork handlers). Rather than "a large number of parameters for all possible situations", another alternative would be to have (1) a call which given executable name and arguments returns an opaque handle (or file descriptor) representing the process to be started (2) a bunch of further calls to set attributes on that handle – new features could add new APIs acting on the handle, or an extensible API like ioctl could be used – if there is a handle to represent the current process, then you only need one API call to set it for the current process or a child to be started (3) finally, a start call which turns the process-to-be-started handle into a running process handle.
> Unix file permissions are shit
The user-group-other model is arguably too limiting. ACLs are a better idea, but then should you use POSIX ACLs or NFSv4 ACLs?
The distinction between primary group ID and supplementary group IDs is silly.
Why must every file have both a UID and a GID? For files owned by a single user, you end up creating a dummy group like "staff" or so on just to obey the rule that every file must have a GID. For shared files, e.g. project files, files generally end up owned by their creator, even though in a business sense they really belong to the project not to whoever created them. It would make more sense if the owner could be either a user or a group, and then also have zero or more non-owning groups associated with it.
In most cases permissions should only exist on the directory, and then automatically apply to any files in the directory. (In most cases every file in the same directory should have the same permission; Unix bases its design on the exception rather than the rule.) Of course, hard links make this impossible, but I think hard links were a mistake.
The executable permission bits actually do double duty as a file type indicator. That's rather ugly. If Unix had explicit file types (rather than just a naming convention of file extensions), then certain file types could be declared to be executable. Executable permission would then mean "you are allowed to execute this if it is an executable" instead of "this is an executable". Stuff like the +x vs +X distinction in chmod would never have been necessary.
> Let's not even get into everything is a file
Unix would have been much better if everything were a file descriptor, rather than having stuff like pid_t. Linux at least is evolving in this direction. Plan9 does it better. Even the WindowsNT philosophy of "everything is a handle" is better than the traditional Unix approach.