Why is there no /usr/bin in Windows?

Usually there should not be any issues. However, you need to watch out for two things:

• Avoid name collisions. In particular, make sure the standard path components (like C:\windows etc.) come first, so no system utility is hidden in the path.
• The PATH variable may not contain more than 8191 characters. So if you have many program folders that you want to add, you may have to pick and choose. If you assume an average path length of 50 characters per program, that gives you space for about 160 programs in the PATH.

The reason this is not done by default is probably simply that the command line is used relatively rarely on Windows, so there was never a pressing need to have everything in the path. Most programs that need to invoke other programs have adapted to this, and use the full path for invocations.

Finally, if you would like to work on the command line a lot, like on Linux, I recommend you install Cygwin. It provides a Linux-like environment on Windows, and lets you work on the command line. It also maintains its own PATH variable, which you can customize without impacting other Windows applications.