I really don't see this working.
Let's think about what has to go in something like this:
- Site designers and Stack Exchange management would have to agree that we need something like this. If there is not any community support for the implementation of this, there is no way you would get this considered, or even looked at. Plus, something like this would need general acceptance from about 95% of Stack Exchange site. You can't just make a major change like this on one site.
- Let's say this does get general community acceptance (which I doubt it would because nothing is really wrong here). Site designer(s) (I'm fairly positive Jin is the only one) would have to come up with the layout, and the developers would have to test an implement it. Now, this is just the general development process that we all know, but having only one site designer doesn't help your cause.
- Finally, SE usually tries out the feature on a few sites at first before totally launching it. Do you honestly think that people are going to write summaries on every accepted answer and closed question? We're all volunteering our time and knowledge to the site (unless you're an employee). I would be really hasty about asking and answering questions if I knew I was going to have to write a summary. Plus, moderators are volunteering their time also. They don't get paid, and from what I'm aware of, I doubt they'd want to do this work either.
I'm not trying to say that anybody is lazy. I'm not trying to say that we don't want to make this community a better place. What I am saying though is that nothing is broken, and if it's not broke, don't fix it.
For more info on what you can do with the current system in place, see ChrisF's answer.