I have not yet tackled PCMCIA or USB peripheral support, the main reason being that I do not own anything which uses either. However, eventually it is desirable to have these working. USB in particular will offer a high-bandwidth networking alternative to PLIP, with the right cable and cross-over thingy.
Both USB and PCMCIA are available as kernel configuration options in the current development kernel (2.3.33 at the time of writing). Debian has PCMCIA utilities in the standard distribution. I look forward to testing this out, but I need to get a PCMCIA card first.
One other potentially useful feature is ACPI support. This is partially available in newer kernels, and a project is underway to provide a complete implementation (both kernel and user-space tools). However, this is undergoing heavy development, and there isn't much point enabling it at present (it has caused hard lockups on my machine, but it did successfully detect the ACPI hardware).