I am generally a Slackware user so I'm used to more or less manually building and installing packages with slackbuild scripts or just using a few trusted repos for some packages like VLC or ffmpeg. I decided to try Gentoo on a new machine to see how it compares and so far it is amazingly fast, but I'm a little confused about emerge. I have built the base system and I have KDE working, and I have sound, but I have rebuilt somepackages 4 and 5 times due to lacking dependencies. A few packages like VLC and ffmpeg have a lot of optional dependencies. I was wondering if there was a command that would just bring these dependencies in and build them. I think might accomplish this, but I'm not sure. Do I have this correct?

Generally you fine tune those optional dependencies with the use flags. There is a USE var in /etc/(portage/)make.conf which is for global flags, and you can set per packase flags in /etc/portage/package.use . That said, it is good, especially when you update world, to add -N (--newuse) option to emerge which will check for changes in use flags and re-emerge/update the packages affected. A typical update command is (if you haven't already read it elsewhere)
Hope this helps a bit.

A simple "emerge <package>" will pull all the dependencies of a given package.

If you think that some dependencies are not being pulled correctly then the fact is that that package is not a dependency. And if something that you think should be a dependency is not, then the big chance is that, more than probably, an USE flag in <package> needs to be changed. But it could also be an USE in a dependency, and not directly in <package>.

If you need to go picking random packages by hand, then you are doing something wrong, and you probably should be rather looking at the output of

and see what's enabled and what isn't.

Yep, emerge <package> will pull in all dependencies you need based on your USE flags. To check if you're missing dependencies you can optionally run revdep-rebuild.

revdep-rebuild is not meant to be a dep-checker of any kind, though it obviously does some basic dep checking before doing its real work just to make sure that something trivial is not missing.

The real revdep-rebuild taks is to check if the ABI is consistent across all your packages and libraries, so that if some random package was built against any-random-lib-1.x, but you upgraded to any-random-lib-2.x, it will continue to work (by recompiling it against the new library).

Dependencies do not work that way, they are in fact checked *before* you emerge the package, much before you have the binary file that's all revdep-rebuild cares about.