What is the effectiveness of having a straightforward diction in a short story?

It's probably more realistic, especially if the story is told in a 20th or 21st century setting.

If you want to know what the difference would be, read a bit of Shakespeare, and then read a bit of DH Laurence.

http://www.online-literature.com/shakespeare/hamlet/3/

http://www.classicshorts.com/stories/rockwinr.html

Hemingway's work is also a good example of straightforward diction and style.

=)