You explore a new command line tool. You have not bothered to read any of its documentation. For that matter, you don't know what the tool might do. An interactive mode is available, a fact of which you may or may not be aware. You decide to type the command with no arguments.

I wouldn't. I might try prog --help and prog -h if I had some clue what the program was, otherwise I'd reach for man prog and/or Google.

What should the tool do?

The default action or whatever the author wants.

What do you expect?

Anything. I have no expectations of what the default would be.

What would annoy you?

I wouldn't be annoyed, except with myself if I didn't bother to RTFM before blindly executing a command.

What would you prefer?

Something sensible, like a default action, if appropriate.