Yes, I've used shampoo as bath soap several times over the years. Never bothered me one way or the other - no "slimy" feeling, and it got rid of bush dandruff!
If we're talking standard grocery store shampoo vs. standard grocery store soap (like Suave vs. Dial), they both probably add plenty of ingredients that aren't needed. While there may be a difference in how the bather/shampooer feels after using it, I can't see any real long-term harm even with daily use.
Comparing a triple-milled bath soap vs. any shampoo, there might be a noticeable difference in how the skin feels.
You don't get the same lathering/life from shampoo, though. That is noticeable and it might take more shampoo to do the same job as soap.