Arch is similiar to slackware in many aspects, but adds a lot on top of that. It has a good binary package manager, which is (almost) as good as apt-get. In addition, it has a source based system called ABS. This system is not as powerful as gentoo, but it works well.
The whole arch system is i686 optimised, so it needs a modern (P2/athlon or above) to run, but runs a bit faster on this hardware.
Its init scripts are simple, and not System V based. They are similar to slackware, but more flexible in some ways. The basic configuration is managed by a central file called rc.conf. It maintains a list of daemons to start, network settings, and other options.
Arch is a rolling distro, so if you update using the package manager, you are always running the latest version of the distro, which is pretty up-to-date. The arch system is quite dynamic, and there are quite a few innovative changes, for example when udev came out, arch was using a very fast system called hwdetect to detect and load modules based on information exported by udev.
But there are some problems. Sometimes major updates (udev, Xorg7) may break the system, so one needs to know how to fix things in an emergency.
To have atleast a part of this post on topic, here is a link to a good kernel compile guide -
*www.digitalhermit.com/linux/Kernel-Build-HOWTO.html
. I used a custom compiled kernel when I was running slackware, but now in arch linux I stick to the stock kernel, and with frequent updates I find myself running the latest stable kernel a day or two after its release.