Is this an ethical use of other peoples' content?

I am planning to create a website that will curate selected posts from around 30 blogs in a specific niche and organize them into a detailed hierarchy that makes it very easy for visitors to access all the content on any given topic or sub-topic. There will also be a range of tools that make it easier to discover related topics within the hierarchy.

The site database will only store the title, the URL and a short excerpt from each contributing post. When the visitor selects a post to read, she will not be redirected to the original post. Instead it will be displayed in an IFRAME below a simple Twitter-style header bar that includes widgets for rating and sharing the post. The branding of the original page and all its links will be left intact, enabling the visitor to navigate directly to the source in a single click.

My goal is not to plagiarize other peoples' content, or even to disguise its origin, but simply to organize it in a manner that yields more than the sum of the parts, together with tools for collectively rating it

Does anyone have any views about the ethics of this approach?

I don't see any problem with this. LinkedIn does a similar thing for any links that are posted by users.

If you make sure that all the stakeholders win from your site existing, then you should be fine. By stakeholders I mean, users and site owners as well as yourself.

My personal perspective as a content site owner is that I wouldn't have a problem with it as long as whole posts from my site weren't copied. If it directed people to read my posts and look at other articles on my site, I would be very happy to have my posts mentioned.

Having said that, I also think it may be wise to get advice from a lawyer if you are in doubt.

There's no problem as long as you are not putting ads in your header bar, Other wise it will be considered as you are monetizing on others work: Copyright infringement.

Alternatively you can ask the authors for permission.