WordPress.org

So what's the advantage of this plugin over the original All in One SEO Pack ?

The usage is simpler as the keywords field is disabled by default. And the WordPress excerpts can be turned of for descriptions.

Can I turn the keywords field back on?

Yes you can. Check out 'Add keywords field to post editing screen' in plugin options.

Why would anyone want to turn of the WordPress excerpts for the descriptions?

If you are doing some custom stuff (for example) with your templates, it might come handy. We need it.

What about the rest of the original plugin functionality?

It's all there.