I know a good bit of the draw of Gentoo is setting USE flags and CFlags, and from my understanding of it, dumping unneeded USE flags from packages ultimately is what sets Gentoo in a league of its own for speed, however...does Portage give an alert if you get rid of a dependency vital to the package in question? The end goal will be of course pre-selecting a list of what I want to install, and ONLY getting the dependencies needed for those items, to ensure maximum speed in loading and executing. Also, is it possible to do the same thing with the kernel, or is this mostly done for me by the compiler? Sorry if these were covered in the Gentoo handbook, I've been pouring over it for a while now and didn't see it. I'd attempted to install Gentoo once from the minimal CD, but due to either a damaged ISO or a bad tg3 driver, it never got off the ground, so I'd like to give it a second go and see if it's got a leg up on my current lean, mean ArchBang setup. Thanks!

If a package is vital, then the USE flag is not used within the ebuild, removing the option to not use it.

USE flags are not dependencies though, they control the compiled functionality, not just that it needs another package.

So for example, if I'm using OpenBox and only ever will, I have no need for the myriad of compatibility/functionality parts of a package pertaining to KDE or GNOME, and thus can dispose of that via the USE flags?

Yes, it's important to know which USE flags do what. I believe all you need to do is add in "-kde" or something similar. You can find a list of USE flags either on the system locally or online at http://www.gentoo.org/

You will have to rebuild the packages with the new USE flags which Portage can do for you.