I'm not crushing dreams or bursting bubbles with this post. I don't think anyone believed they'd ever run around with actual light sabers. Still, let's take a look at what separates the impossible from the preposterous.
1. They need power.
And lots of it. Even dinky little cat toys need batteries. In order to strike someone down – even if they do become more powerful than you could possibly imagine – you need a large amount of power to generate an intense amount of heat. The best case scenario would be attaching the light saber to a cart that can carry around the power source and then conducting battles the way you would play bumper cars with grocery store shopping carts.
More likely you'd have to plug your light saber into a wall. Not only would that make them less portable, it would severely limit their coolness factor. Jedi warriors would be demoted from noble, mystic space samurai to a couple of idiots dueling with electric turkey carvers.
2. They also give off power.
And just to suck some more cool out of the picture, imagine that the two idiots wearing oven mitts while handling the electric turkey carvers. Lasers may focus most of the power into heat and light going one direction, there is still a lot of heat being put off from the device itself. Perhaps a Jedi trick could be invented that would allow the Jedi to duel without tossing their light sabers from hand to hand, exclaiming, "Hot! Hot! Hot! Hot! Hot!"
Still, even if the Jedi could keep their hands from sizzling like bacon, the sabers themselves would overheat unless they were equipped some kind of device that would cool them down. These cooling devices are also usually pretty bulky. So not only is the saber plugged into a wall like a turkey carver, it's going to have to be swaddled in bags of frozen peas to keep from overheating.
3. They could be fended off using a compact.
The reason mirrors are so valuable to those who are vain, those who are worried that they have food on their face, and those who need to hunt vampires, is light bounces off of them pretty much intact. To say that a light saber's blade would be deflected, or reflected, entirely would be overstating the case. Even the best mirror doesn't reflect perfectly. However, getting your own light saber in the eye because someone was checking their lipstick has to be even more ignominious than killing the younglings. At least being evil isn't embarrassing.
But all of those are just practical problems. There's always the possibility of more compact power systems and smaller cooling systems. And mirrors don't reflect everything. The major problem with light sabers is this: Light keeps going.
Light goes from the sun to the earth in a vacuum. The light from a regular bulb travels across rooms. A light of such high power that it murderizes people will not stop three feet from its source without something concrete to stop it. The most likely way to stop it would be a mirror, but that not only brings us back to problem number three, it necessitates a device that will hold a mirror three feet from the base of the saber.
That is, assuming that a light saber is just photons. Some have floated the theory that a light saber isn't a laser, it's a directed plasma. Plasma is gas that has been ionized. The electrons of the molecules of gas have been tugged away from their protons. The plasma theory has problems, though. The best way a plasma can be directed is using magnetic fields as well as the requisite mirror at the top of the saber to cap the saber. The magnetic fields would have to enclose the entire blade of the light saber.
Some readers may be asking why these things make light sabers impossible. To be honest, they don't. It would be possible to use a mechanical apparatus to suspend a mirror three feet over a light saber, and to use a mechanical apparatus surround the blade of that saber with machinery that would create magnetic fields. It's just that those things would not really be light sabers. Generously, they could be called swords. More accurately, they would probably be called . . . sticks. They might be made of plastic, or wood, or even metal. But they'd still be called sticks.
Really takes the glamour out of things.