Something benign, but useful :)

To be told, in brief, what it expects.

Being told: "You didn't give the appropriate arguments; type 'xyz --help' to get help".

Usage: 'xyz [-a] [-b] [-c pqr] -u user file [file ...]
Type ? [opt] for extended help; return to exit; or enter your command 
+line now
xyz>
[download]

Even better if at that prompt I could use uparrow to retrieve the last N command lines I've used with this program.

For ultimate brownie points, allow me to enter a partial command line followed by ? and it displays the help for the preceding argument element, and returns to the prompt with the input buffer pre-filled with the command as was when I hit return. minus the ?.

Eg.

Usage: 'xyz [-a] [-b] [-c pqr] -u user file [file ...]
Type ? [opt] for extended help; return to exit; or enter your command 
+line now
xyz>-a?
Option -a: output all information
xyz>-a -c?
Option -c pqr: set the c... to the (required) value 'pqr'
xyz>-a -c pqr -u?
Option -u user: (required) specify the user id for ...
xyz>-a -c pqr -u fred ?
You must specify one (or more) filenames (wildcards allowed) to be pro
+cessed.
xyz>-a -c pqr -u fred this*.txt that*.txt theOther.txt
(processing)...
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Ie. I get just the help I need, when I need it, without having to start over, start a new session, or wade through screenfuls of stuff trying to find it.