Windows Command Prompt: dir + what option shows only Directories and not files?

All / (switches) in Windows don't have any filter purposes.

I know in Linux it is - (options).

At a Windows command prompt, how can I show only Directories and not Files?

This is done by filtering by attributes. dir [somedir] /ad will show all entries with the "directory" attribute. It also shows junction points.

From dir /?:

Displays a list of files and subdirectories in a directory.

DIR [drive:][path][filename] [/A[[:]attributes]] [/B] [/C] [/D] [/L] [/N]
  [/O[[:]sortorder]] [/P] [/Q] [/R] [/S] [/T[[:]timefield]] [/W] [/X] [/4]

  [drive:][path][filename]
              Specifies drive, directory, and/or files to list.

  /A          Displays files with specified attributes.
  attributes   D  Directories                R  Read-only files
               H  Hidden files               A  Files ready for archiving
               S  System files               I  Not content indexed files
               L  Reparse Points             -  Prefix meaning not

From Microsoft's TechNet - Dir:

If you use /a without specifying Attributes, dir displays the names of all files, including hidden and system files.

The following list describes each of the values that you can use for Attributes. Using a colon (:) is optional. Use any combination of these values, and do not separate the values with spaces.

d Directories

h Hidden files

s System files

l Reparse points

r Read-only files

a Files ready for archiving

i Not content indexed files

- Prefix meaning "not"