"Isn't it interesting?" is either to confirm your opinion that it is interesting (=it's interesting, isn't it?), or to verify the person's opinion that it isn't interesting... because you thought it was interesting. "Is it interesting?" simply means you want to know.

When you use 'Isn't it interesting?',you expect the person to say 'yes', when you use 'Is it interesting?' , you are asking the person wether it's interesting or not.

Isn't this question interesting? How can it not be. :P

Sorry, i don't know why spaces aren't working. I will try again with some html code, forgive me if it doesn't work again:

I would use "isn't it interesting?" if someone does something that suggests a thing that should be interesting may not be interesting after all...

Me: Hey, are you watching that really cool TV show you were waiting for right now?
You: Nah, I don't feel like watching it right now.
Me: Why not? Isn't it interesting? (like you are asking because you are shocked that it may NOT be interesting)

Or like the people said below, to confirm...

You: Oh man! I just read that book you loaned me. I couldn't put it down!
Me: I know, isn't it interesting?
You: Yes, it really is.

"Is it interesting" is something you would ask when you have absolutely no indication as to whether something may be interesting or not.

You: Hey I'm watching that TV show they keep talking about on commercials.
Me: How is it? Is it interesting?
You: It's alright. Not what they hyped it up to be.

or

Me: I just got this book on hypnotism.
You: Is it interesting?
Me: Yeah it is. I might have to loan it to you.

I would use "isn't it interesting?" if someone does something that suggests a thing that should be interesting may not be interesting after all... Me: Hey, are you watching that really cool TV show you were waiting for right now? You: Nah, I don't feel like watching it right now. Me: Why not? Isn't it interesting? (like you are asking because you are shocked that it may NOT be interesting) Or like the people said below, to confirm... You: Oh man! I just read that book you loaned me. I couldn't put it down! Me: I know, isn't it interesting? You: Yes, it really is. "Is it interesting" is something you would ask when you have absolutely no indication as to whether something may be interesting or not. You: Hey I'm watching that TV show they keep talking about on commercials. Me: How is it? Is it interesting? You: It's alright. Not what they hyped it up to be. or Me: I just got this book on hypnotism. You: Is it interesting? Me: Yeah it is. I might have to loan it to you.