idea, in the sense of a concept
It could be that you don’t like the actual activity of dieting, so you make a general statement of your dislike:
I don’t like dieting.
Or it could be that you don’t like the entire concept of dieting:
I don’t like the idea of dieting because you should already be eating a healthy balance of foods, including fruits and vegetables, so you shouldn’t need to go on or off of a diet. If you’re already eating foods that are good for you all of the time, there is no ‘diet’ for you to go on or off of.
General statement: “I don’t like them talking behind my back” or “I don’t like it when the two of them talk about me behind my back.”
“I don’t like the idea of them talking behind my back” doesn’t necessarily mean that you know whether the two of them have been talking about you behind your back or not. It means that you don’t like the concept of them talking behind your back. (But the mere fact that you mention it implies that you either know or suspect that they have been talking behind your back.)
Perhaps the concept doesn’t bother you so much, but you would prefer that they speak to you directly. Then you might say, “I don’t mind the idea of them talking behind my back, but I wish they would be more honest and open about their feelings.”
But really, there’s only a subtle difference between saying, “I don’t like the two of them talking about me behind my back” and “I don’t like the idea of the two of them talking about me behind my back.” Don’t try to overanalyze it. When someone says “I don’t like the idea of,” it doesn’t necessarily mean they are consciously thinking of it as a concept. It’s just an expression that makes it more emphatic.
“I don’t like the idea of you sneaking over to see Shirley like that.”
Likewise with “I don’t mind.” Saying “I don’t mind the idea of” makes it more emphatic.
“I don’t mind the idea of you going over to see Shirley, but I wish you wouldn’t sneak around.”
I don’t like you talking to me that way.
more emphatic:
I don’t like the idea of you talking to me that way.
I don’t mind you going out all the time, but I wish you wouldn’t drink so much.
more emphatic:
I don’t mind the idea of you going out all the time, but I wish you wouldn’t drink so much.
(Actually expressions like “the idea of,” “all the time,” and “so much” all add emphasis. You could simply say, “I don’t mind you going out, but I wish you wouldn’t drink.”)
Hope this helps.