Loading a com line with capacitance is perfectly acceptable, if it's consistent with the signaling standard. A low impedance, terminated transmission line medium, like RS422/485, with moderate bandwidths (say 115.2k), can tolerate a lot of capacitance, and probably should -- because you don't want to leave excess bandwidth in your circuit. CAN I'm not too sure about, but it's probably similar.
Some examples where it doesn't work as well:
- Weak logic outputs (TTL/CMOS, like a parallel port), where the edges are significantly slowed (which causes problems for edge triggered signals), and the voltage levels get shunted by any loading.
- Switched states, typical of RS-485 multimaster, USB and others: the unbalance and varying impedance don't filter well. In some cases, you can deal with it (like RS-485 usually), but others just aren't any good outside of a well shielded cable.
- High bandwidth signals (USB2.0, LVDS, etc.), where the signal itself just can't tolerate much [lumped] capacitance, which means you don't have anything for the CMC to work against.
BTW, as long as the pin driver is capable of driving a characteristic impedance, the capacitance of the cable is a non sequitur.
Tim