DOUBLE EXPERIENCE COST
This is the great equalizer built into the game. Without a master, a Force using character must spend twice as much experience raising Force skills as they do raising other skills. This ensures a very slow advancement in abilities—especially if the player tries to balance raising 'regular' skills along with Force skills. In the early stages of my campaign, we had two main Force users. Arianne's skills grew slowly, as her player wanted to keep her balanced (and relevant) in non-force areas (namely piloting and command). Bob the Tusken was definitely the more Force-centered of the two, spending almost ALL his points on Force related stuff. However, this severely limited him in other areas (But then, that was kind of the charm of Bob as a 'primitive' among the group). So there's balancing factor number one: experience point cost and the need to keep balanced with regular skills or risk becoming a 'one trick pony'.
RESTRICTION OF FORCE POWERS
Though this is one thing I never really used in my campaign, it is still a valid method of controlling how powerful force users can get. Though there are three force skills (Control, Sense and Alter), there are numerous powers that must be learned in order to make use of them. These powers include such things as telekinesis, enhance senses, etc.. Since most campaigns do not have Force masters actively instructing a character, the GM can pretty much pick and choose which powers a Force user gets and when they get them. Thus, if you don't like a power, you can choose not to let a character learn it. With the level of trust I had in my players, however, I only had to exercise this option once—and that was with a 'new' force power introduced in the Zahn Trilogy. In actual game play it turned out to be too powerful, so I had to invoke my GM power to nix it. (For the curious, this 'banned' Force power was Enhanced Coordination).
THE SKYWALKER RULE
This is something I implemented as a house rule when the characters started to get higher-level Force skills (climbing into the 5-6D range). Since I love the original trilogy so much, I made up my mind early on not to dilute the emphasis of Luke's 'heroes journey'. He was the son of the 'chosen one'—and the one destined to bring about the end of the Empire and the rebirth of the Jedi Order. It just would not feel right to me if, by the time that destiny began to roll around, he was totally outclassed by the PC Jedi in my campaign. I felt (and still feel) that having PCs who were better than Luke would diminish the overall story of the saga. Therefore, I let my players know up front that there would be a completely arbitrary Force skill limit set by me that they could not pass: they could not raise their Force skills any higher than 1D less than whatever Luke's Force skills were at the time. Thus, if Luke's Control skill was 'currently' 7D, they could have a maximum of 6D in their Control skill. Period. End of story. Oh sure, there were a few gripes and groans, but that was just how it was in my campaign.
PARRYING AND DODGING
This was a small house rule that wound up having a big effect on how the Jedi handled themselves in battle. I actually touched on this in my post about dodging, but I'll elaborate here. As the Jedi in my campaign began to get more adept at using lightsabers in combat, they (unsurprisingly) tried to use them more and more. Often they would charge down a hall, parrying (or dodging) as they went—closing the distance with their enemies to hand-to-hand where they are VERY deadly. That quickly began to irk me, as it just didn't seem real or fair that they could charge right into the TEETH of a barrage, and still get their full defensive bonus. Therefore, I simply decreed that you cannot do that—at least not at full speed. You can zig-zag your way by dodging or advance slowly if parrying, but you can't go full speed. This meant that it took a while to close with an enemy—meaning that charging straight in wasn't ALWAYS the best plan.
PARTY BALANCE AND DYNAMICS
This is the most crucial (and most elusive) way to ensure that Jedi do not unbalance a campaign or steal the show. And I will admit that it is the one I have to struggle with a lot. The argument I see leveled against D6 Force users most often is that they are show stealers. That at high level, they become the only focus of attention and everyone else in the group is just there to 'mop up' after they're done. If not handled correctly, this could easily be the case.
In the second year of the Vermillion campaign this situation began to develop. At this point, some of the other players called it (good naturedly) the 'Arianne and Bob' show (after the two main Jedi in the group). More often than not, a lot of the action centered around these two. But thinking back on it now, I am not entirely convinced that was because they were Jedi. In fact, I think it was because they were the two most experienced players in the game. Steve and Martin were both several years older than myself and most of the rest of the players. As such, they were quite good at (consciously or subconsciously) guiding the story back to their characters. They were (and are) both very forceful personalities as well—ensuring that a young GM like myself gave their 'squeaky wheels' the grease. Don't get me wrong, though—neither of them were complainers, they were just a lot more vocal and confident in their actions.
On the flip-side, the rest of the players had a bit less experience, and thus were willing (at first) to defer to the more experienced guys. As Steve2 said to me recently: "At the time, I wouldn't have been able to lead the party, anyway—I wouldn't have known what to do in most cases." This was both good and bad. Good because Steve and Martin did have a lot to teach in the way of roleplaying. But bad because all players, no matter how well meaning (and I include myself in this) are ultimately selfish. Players ENJOY having their characters in the spotlight (I know I do). I can't fault anyone for this, but it does mean that you don't always go out of your way to make room for other players.
As a relatively new GM (this was what I consider my first REAL campaign) I had never really had to deal with party dynamics before, so it took me a while to recognize and react to the problem. It was the 'Rogue Squadron' incident that really tipped me off to it. Luckily, I feel that I got to it before it got out of hand. What I realized was that it is the GM's DUTY to try and make sure everyone has a moment in the spotlight.
Again, I don't think that the spotlight was, in this case, stolen because Arianne and Bob were Jedi. Rather, they stole it as players and just happened to have Jedi as characters. HOWEVER, there is still an important lesson here to GMs. People who do, through the powers of their Force users, try to 'steal the show' can best be kept in check by a watchful GM who makes sure not everything can be solved by a Lightsaber or Telekinetic push.
And finally, I should point out something that my players have discovered through play—that no matter how powerful your character, they are ALWAYS more powerful when working WITH someone else. In the case of Jedi, they have plenty of weaknesses and drawbacks when fighting alone or alongside other Jedi—lack of a real ranged attack is glaring among these. Likewise a skilled gunman working with a Jedi intercepting shots aimed at their duo can lay down a deadly barrage without having to worry about dodging (in fact, Padme and Anakin kind of demonstrated this at the end of Attack of the Clones). When the whole team is working together...that's when the 'unbeatable' thing starts to come into play, but to me as a GM, that's a thing of beauty. I remember one instance of this during the Vermillion campaign—when the PCs worked together to take out their rival party of 'Evil Heroes'. Holy crap. Gunmen laying down covering fire. Jedi working their way in close. It was awesome.
So, there you have it. I feel that there ARE tools within the D6 system to ensure that Force users are balanced. But I also feel that the GM and his handling of the players is probably the most single important aspect of that balance. I would much rather have the responsibility of doing that rather than having a game system that did so through 'watering down' Force abilities—which are SUPPOSED to be amazing and fantastical.