The concept of a
lightsaber is, in itself, a most unlikely and
fictional
anomaly.
The first theoretic negation of a lightsaber comes from rather common sense. If the blade is composed of a (does quotation fingers) laser, what keeps this weapon from simply reaching infinitely into space as a beam of light actually would by nature? What is actually holding this beam in place?
One theory that is more feasible than simply saying that the lightsaber just "is" would be that the sword is, in actuality, many beams of light. These beams could all have an incredibly close wavelength to each other and be angled slightly towards the center of the handle just shy of a 180 degree angle. When and only when these beams strike each other, the wavelengths could resonate with each other to form a consistent beam. After the point of focus is passed, the beams would again become ineffective. In this way, the lightsaber could just maybe (albeit unlikely) exist as a focus weapon instead of a beam weapon.
Another problem is, of course, if the beams are made of light or other energy, how can one saber stop another? In every Jedi duel, there are almost as many blocks as there are attacks. One brilliant flash later, both fighters emerge unharmed. If the weapon were a true beam or even focus weapon, no matter how powerful the beam, light cannot act as a solid matter beyond the idea of a photon which has a mass much too miniscule to even be reasonably measured by the most modern equipment. This argument is not even taking into account the fact that there is no modern mobile power device capable of maintaining such a high output of energy. This argument finds its fallacies in the "far far away" clause, implying that technological advancements made in the setting of the Star Wars movies have no correlation to the technology of our time and place.
Another interesting point to the lightsaber is the fact that it would be quite easy to weild, requiring almost no strength to lift or swing. However, as any swordsman will tell you, a pivotal aspect of choosing or making a sword is its balance. The typical point of balance for a sword is slightly above where the blade meets the handle. If the blade of a lightsaber has no weight, being composed of energy, the balance would reside at the center of the handle, making the saber very difficult to become accustomed to and/or use.
Summarily, the idea of a lightsaber is, in itself, self-defeating. Although ultimately a very cool device, it is destined to forever remain in the annals of fiction.