Ok, so I started reading through your post =( and I'll answer some of your specific questions.
The optdepends aren't really like USE flags (I tried Gentoo a few weeks before I got fed up and discovered Arch, haven't looked back). The handling of optdepends at this point in time is pretty minimal, but there are discussions on the bugtracker and mailing list about things to be done, someone just needs to take the initiative.
You can use unstable packages from the [testing] repository. And unlike my experience with Gentoo, you can choose a single [testing] package if you want, whereas with Gentoo I recall being forced to go all or nothing on the stability. It's also quite easy to manually update a package if the devs don't quite have the time. It usually requires not much more the downloading the PKGBUILD and changing the version number variable =)
Every packages has 0 or more dependencies and make dependencies. You may not want gcc on your system, but obviously you'll need it as a make depend to build a package. In fact, I don't think gcc even shows up as an explicit makedepend anyware. In any event, it's quite easy to remove uneeded packages with
pacman -Qdt # list packages which were installed as dependencies but not currently a depend of another
sudo pacman -Rcs $(pacman -Qqdt) # remove the packages, all their dependencies, and anything that is dependent on them
In fact, the popular AUR package "yaourt" will automatically prompt you for a "-Rcs" when you specifiy "-Qdt"... I'll explain further if you ask =)
Arch packages are .tar.xz (used to be .tar.gz). By default when you choose to install a package, it's saved on your computer under /var. When you build your own package, the source code is downloaded directly from upstream's website, it is not stored on your computer. As for handling the .tar.xz's, "pacman -Sc" will remove all but the latest version of each package that has accumulated. "pacman -Scc" will remove all cached packages. And there are options in pacman.conf to choose to not store the pkg files at all. I'd say 1 - 2 gigs is normal with the default settings, but you can _easily_ change the behavior.
Arch's documentation (Wiki) is on par with Gentoo's. In fact the Arch wiki and Gentoo wiki are two of my most used sources of info. The message board is pretty slow, but the mailing lists get a lot of activity. The IRC channel is also very popular (usually around 850 users).
I absolutely love Arch. The feature that apt-get has that pacman doesn't is that it can handle changes in configuration files. pacman has a very simple algorithm for how it handles changes in config files. If the config from the old version and new version is different *and* your config is different from the both of them, it will store the new file with a .pacnew extension and tell you to take a look. Checkout "man pacman" it's very clear.
You will learn a lot about _*nix_ with Arch. Not just the distribution itself. It has just the right amount of mandatory DIY. And unlimited optional DIY. You don't even have to do get your hands dirty but you can just read the PKGBUILDs to see a lot about how packages are set up and compiled.
Oh, and Arch's installer is perfect - the devs have done a great job with it. Besides the fact that the same image can be used for a CD or a USB without any extra work. It also has both x86_64 and i686 on the same image (if you want). The core_dual image encompasses all of the other images in ~600 mb. The only reason the others are available are for size restraints.
You didn't ask, but Arch packages also have minimal (usually 0) non-vanilla patches. (A plus in my opinion)