In again perusing various Star Wars RPG forums, I have recently been pondering another 'complaint' that a lot of people seem to have about the D6 system. The consensus seems to be that Jedi, particularly 'high level' jedi (those above 4 or 5D in their skills) are unbalanced and practically invincible. Now, while I agree that there is a 'tipping point' on Force skills that takes them from 'useful trick' to 'near superpower', I do not feel that it makes the game unbalanced or 'unrealistic'. My reasons for this belief are as follows:
Within the Star Wars movies we are shown that fully trained Jedi ARE pretty mega. In Episode I, for instance, the Trade Federation guys at the beginning receive word that there are TWO Jedi on their ship and one of them chimes in with "We will not survive this." Yes. Some of that is their own cowardly nature and the whole intimidation factor of the Jedi, but we are shown that two Jedi CAN handle a lot of opposition all by themselves. And yet in the same segment we see that Qui-Gon and Obi-Wan actually have to flee when they hit too stiff of opposition.
Strangely enough, when the Force users in my campaign started to get powerful, they experienced much the same kind of dynamic as shown in the prequels. And mind you, most of my campaign took place BEFORE the prequels ever came out. In a hand to hand fight, or versus a relatively limited number of opponents, the Force users were indeed pretty much untouchable. But when facing highly skilled opponents, or masses of troops (squad sized or larger) a lone Jedi (or even pair of Jedi) was no match.
It is, to me, one of the selling points of the D6 system that even 'super powered' characters can be brought low by bad luck (a blown roll) or by overwhelming numbers. It is realistic and heroic at the same time, and gives even a high-power character reason to be cautious. And it all exists within the game mechanics of the system—or at the very least within my interpretation of it.
The combined fire rule is one of the main tools a GM can use to level the playing field. Say you have a squad of eight stormtroopers with 5D blaster skills (the average in my campaign). They engage a Jedi by firing a volley of eight shots. Mechanically, this means you take the lead shooter's skill (5D) and add +5 pips to it for each additional shooter (because each shooter has a 5D skill—if they had a 4D skill, you'd add +4; 3D you'd add +3, etc.) This means that as a squad, the troopers would be rolling 5D+35—producing an average roll of 52. Say your Jedi has a 7D Sense skill (the skill used to parry blaster shots). Even if he does a full parry (doing nothing else that round but blocking, which adds 10 to his roll) his average roll would be a 35. He'd get shot.
At that point, you also figure in the damage system in Star Wars. The average character has a strength of 3D to resist damage. The average trooper blaster does 5D damage. Which means that our Jedi would at the very least take a wound—which would further impair his ability to fight and defend himself. Even if the character were wearing typical armor (which adds 1D to their strength roll to resist damage), odds are they'd still take a wound. Yes, a Jedi can try to control pain from his wounds, but that is yet another action he must take—which lowers his defensive ability again until he is successful... and he'd be doing so in the face of yet another volley from the troopers.
So there you go. Game mechanic-wise, eight troopers can (in the right circumstances) be a match for a single Jedi. The trick for the Jedi is making sure he never faces the troopers in those circumstances. It works in the game and it works from a sense of cinematic and thematic realism, too. Just look at the Jedi generals during Order 66. That was the TRUE GENIUS of the Emperor's plan, to put unsuspecting Jedi into the crosshairs of a number of troops. Most of them didn't even see it coming. Those that did, didn't last long under a sustained volley.
When you DON'T have masses of troops to throw at a Force user, however, there are yet other ways to defeat them (and again, this is borne out both in the movies and in game mechanics). Area effect weapons neatly bypass the Jedi's ability to block or deflect pinpoint or melee attacks. Flamethrowers, Gasses, Grenades, Explosives—all of these mean that the Jedi is going to have to rely on their dodge skill to avoid—and even if they successfully dodge, odds are they are going to take SOME residual damage from an AOE effect (shock from a blast, burns from a flamethrower, etc.). This will, of course, further hamper their ability to defend themselves on subsequent rounds. Just look at the tricks Jango Fett used versus Obi-Wan and Mace Windu—explosives, flamethrowers. They work.
What if you don't have troops or area effect weapons? Well, then things are difficult. If you can, you should flee (just like Jango did on Kamino after he'd used all his tricks). Again, its things like this that thematically support Force users being difficult to handle within the Star Wars universe. It isn't a flaw in the system, its a story element that GMs have to learn to handle.
I suppose the REAL wrinkle in all of this comes with the use of Force Points. I will admit that as a GM I both love and loath these things. If used cinematically, they can provide for some really incredible moments, if used mechanically, they can turn a character (Force user or not) into a tank. But these too have their limitations. First of all, a character only has so many of them—even high powered characters cannot use them indefinitely. Secondly, major NPCs have Force points, too—and can use them to essentially negate the advantage. So as with many things, a GM needs to consider the power levels of his characters and make challenges that will fit them. If you truly want to push a character to their combat limits, then you have several options: a long series of encounters with large numbers of grunt troops or encounters with highly-skilled NPCs similar in skill-level to the characters. I find a mix of these things to work best.
All of that having been said, when you get to 'epic' level characters (above 7D), a GM shouldn't gear his ideas solely towards combat. Presenting players with moral choices, challenges and other difficult decisions with consequences place emphasis on roleplaying and not roll-playing. By this time, most combats SHOULD be easy for the Jedi—but combat really isn't their mandate. They're peace-makers, which makes things interesting and difficult in its own right, as they can't simply 'kill them all and let god sort them out'.