In the beginning, which is to say the early days of Bell Labs Research Unix, life was simple and there was only /bin. Soon afterwards that disk ran out of space and we got /usr/bin (and all of /usr), and some people might even have put /etc on their $PATH. When UCB released BSD Unix, they added /usr/ucb as a place for (some of) their new programs and put some more useful programs in /etc (and at some point there was also /usr/etc); now you had three or four $PATH entries. When window systems showed up, people gave them their own directories too, such as /usr/bin/X11 or /usr/openwin/bin, and this pattern was followed by other third party collections of programs, with (for example) /usr/bin/mh holding all of the (N)MH programs (if you installed them there). A bit later, SunOS 4.0 added /sbin and /usr/sbin and other Unixes soon copied them, adding yet more potential $PATH entries.

(Sometimes X11 wound up in /usr/X11/bin, or /usr/X11<release>/bin. OpenBSD still has a /usr/X11R6 directory tree, to my surprise.)

When Unix went out into the field, early system administrators soon learned that they didn't want to put local programs into /usr/bin, /usr/sbin, and so on. Of course there was no particular agreement on where to put things, so people came up with all sorts of options for the local hierarchy, including /usr/local, /local, /slocal, /<group name> (such as /csri or /dgp), and more. Often these /local/bin things had additional subdirectories for things like the locally built version of X11, which might be plain 'bin/X11' or have a version suffix, like 'bin/X11R4', 'bin/X11R5', or 'bin/X11R6'. Some places got more elaborate; rather than putting everything in a single hierarchy, they put separate things into separate directory hierarchies. When people used /opt for this, you could get /opt/gnu/bin, /opt/tk/bin, and so on.

(There were lots of variations, especially for locally built versions of X11. And a lot of people built X11 from source in those days, at least in the university circles I was in.)

Unix vendors didn't sit still either. As they began adding more optional pieces they started splitting them up into various directory trees, both for their own software and for third party software they felt like shipping. Third party software was often planted into either /usr/local or /usr/contrib, although there were other options, and vendor stuff could go in many places. A typical example is Solaris 9's $PATH for sysadmins (and I think that's not even fully complete, since I believe Solaris 9 had some stuff hiding under /usr/xpg4). Energetic Unix vendors could and did put various things in /opt under various names. By this point, commercial software vendors that shipped things for Unixes also often put them in /opt.

This led to three broad things for people using Unixes back in those days. First, you invariably had a large $PATH, between all of the standard locations, the vendor additions, and the local additions on top of those (and possibly personal 'bin' directories in your $HOME). Second, there was a lot of variation in the $PATH you wanted, both from Unix to Unix (with every vendor having their own collection of non-standard $PATH additions) and from site to site (with sysadmins making all sorts of decisions about where to put local things). Third, setting yourself up on a new Unix often required a bunch of exploration and digging. Unix vendors often didn't add everything that you wanted to their standard $PATH, for example. If you were lucky and got an account at a well run site, their local custom new account dotfiles would set you up with a correct and reasonably complete local $PATH. If you were a sysadmin exploring a new to you Unix, you might wind up writing a grumpy blog entry.

(This got much more complicated for sites that had a multi-Unix environment, especially with shared home directories.)

Modern Unix life is usually at least somewhat better. On Linux, you're typically down to two main directories (/usr/bin and /usr/sbin) and possibly some things in /opt, depending on local tastes. The *BSDs are a little more expansive but typically nowhere near the heights of, for example, Solaris 9's $PATH (see the comments on that entry too).