Pressure sensitivity
The closest thing to actually drawing
for real today comes from using a graphics tablet.
Xara Xtreme just made using a graphics tablet even more realistic though as now you get the advantage of pressure sensitivity.
What that means is that when you are using the Brush Tool you can alter the size and/or the transparency of the brush by applying pressure to the pen.
Xara Xtreme allows you to choose whether you want the pressure to affect scaling or transparency - or both, or neither.