Michio Kaku (physicist guy) once made a TV special on how you could make a realistic working lightsaber, and his plan involved a telescoping ceramic blade heated to unreasonable temperatures. In that instance, it would certainly make sense for physical objects to resist, though still be melted by, a lightsaber.
However, if you're going by the crystals and contained plasma beam thing, it would start to get weird. No matter how hot the lightsaber was, it wouldn't cut instantly through everything. However, a plasma is a gas (sort of), even if contained by a magnetic field or whatever. So if you tried to shove a lightsaber through something, the magnetic field would pass through the material, and the plasma would be compressed. That would heat it up more, and it would start to melt through whatever you were cutting, but after a certain point, you'd no longer be able to push, as you no longer have the physical strength to compress the plasma further.
As for lightsaber combat, I'm pretty sure it's mostly down to the magnetic fields. Blaster bolts are obviously not lasers, but bits of plasma, and so would interact with the lightsaber's magnetic field. Dueling with a lightsaber, your magnetic fields would collide with each other. And then vibroblades or whatever General Grievous' droid pikemen have are quite obviously electrically charged, and thus interacting magnetically as well.
