The main difference between a Traditional Play script and a Readers Theater script is - the RT script is written in such a way that it doesn't require any additional elements to help communicate the story to the audience.

Example: 

In a Traditional Play scene a costumed "old grandma" character might be asked to speak the line: "I wonder where I put my glasses". Then she might walk across the stage to a kitchen table, sort through props set on the table, find the glasses, then exclaim, "Oh, here they are!" So, to communicate the above scene we've used costumes (and possibly make-up) to make the actress look like an "old grandma", we've used stage movement (blocking), a set piece (table), props (assorted clutter - dishes, magazines, pieces of mail, etc. on the table), and, of course, the glasses themselves.

In the RT version of that same scene, the old grandma stands facing the audience. She holds her script in her hands or places it on a music stand in front of her. Another reader, a narrator, stands behind her own music stand or sits on a stool nearby. The grandma might wear a little "granny cap" and/or a sign around her neck that says "GRANDMA" (but this isn't truly necessary, because the script should be written in such a way that NO costumes or props are needed to communicate what's happening in the scene).

NARRATOR: Once there was an old grandmother who lost her glasses.

GRANDMA: It's true, it's true. I am an old grandmother and I do seem to be losing everything these days! Now I wonder where I put those glasses...

NARRATOR: The old grandmother searched everywhere. Finally, she walked to her kitchen table and started looking through the clutter. The table was covered with dishes and magazines and dusty envelopes.

GRANDMA: Oh dear, I've just got to clean up this mess. Now let me see... Well, would you look at that - here they are. I've found my glasses!!!

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And there you have the difference in a nutshell.  It's not difficult to understand why teachers (and students) are not intimidated by RT.  No special drama training is necessary.  There are no endless hours spent assembling costumes, props, and set pieces.  Nobody is asked to memorize pages and pages of dialogue. 

Does RT take the place of Traditional Theater?  Of course not.  It's just another way to involve students in a performance, the telling of a story, or the examination of a theme.   It's also a fantastic way to teach important facts and concepts - and to motivate reading.  As I've said before, "It doesn't get any better than that..."