Well, I am as guilty as anyone for focusing on Adobe Lightroom for photo processing and metadata editing as if it were the only tool in the world. Aperture by Apple offers professional features with the simplicity of iPhoto. And, the features offered for adding keywords and metadata are comparable to Lightroom.
This post on Light Stalking explains how to add metadata and assign ratings in Aperture. I like the search functionality as well as the ease of navigating and customizing the metadata panel. Also, the price of Aperture is quite nice at $79.99.