The biggest thing to mention is of course that a filter attenuates certain freqiencies while leaving others. I'd say next is filter order; how steep the rolloff is. A tie-in for this is that there is no 'brick-wall' filter so there will always be some response from the stopband.
Filter type is also important. Lowpass and highpass are the basics, bandpass and bandstop are pretty powerful shaping tools. Perhaps a little bit about notch filters and their surgical accuracy.
For synth work, topology is probably an important topic. The first that comes to mind is the Moog ladder, you can barely start to talk about subtractive synthesis before hearing about Moog. Sallen-Key is also a classic common topology and is used often by Korg. The other big one is state-variable. This would be a good place to mention 'Q' and resonance. Perhaps a bit about which filters can double as oscillators.
Phase response, I'd say, should get only a passing mention. One: there's not much we can do about it and two: it isn't a big deal unless you're mixing it with dry or another filter with the same cutoff.
Envelope control could be a good topic too. A bit about how the harmonic content/frequency response of instruments evolves as the note decays. Perhaps a bit on how to imitate it.
Wow, that was longer than I thought it'd be. :what:
Hope this helps.