Should players be able to play as Jedi in MMOs like SW:G or Bioware's upcoming game? I think SW:G originally had the right idea: allow players to become Jedi, but make Jedi rare (i.e., not available to all).
As Stéfan points out at MMOCrunch, the first Star Wars: Battlefront was fun without playable Jedi. It sold well enough to earn a port, proving there's a market for Star Wars games that aren't Jedi-centric. In fact, some players (like Stéfan and I) feel that the first SW: Battlefront is actually more fun than the sequel.
On the other hand, players do enjoy being Jedi when not everyone is a Jedi.
SW:G proved, once again, that people generally enjoy lotteries in games. The possibility of unlocking a Force-sensitive slot was like the possibility of looting a Legendary item in Diablo 2: epic, near-unattainable goals raise excitement while players focus on other goals.
The problem with the pre-NGE game wasn't that not everyone could be a Jedi. The problem was that, six months after release, not a single Force-sensitive slot had been unlocked. The basic idea was right, but it was executed poorly.
The trick is finding a way to ensure some Jedi slots are unlocked within the couple months, yet keeping Jedi numbers to a reasonable limit even after years of play.
I might be wrong, but I don't see how that's possible through an achievement system (i.e., players can find how to acquire a slot and work toward it). The main problem is that sort of system encourages grinding... the effective bypassing (waste) of content to reach a goal; the player is blinded to the journey in pursuit of the goal. Even if the content is fun and worthy of enjoyment, players will still rush through it and miss its appeal because they perceive playing a Jedi as the core game.
Some degree of chance is necessary. Unlocking a Jedi slot doesn't have to be completely random, but there has to be some uncontrollable (and fair) element.
Regardless, I would place a hard cap on the number of Jedi per server, just in case. Designers can't predict everything.
It occurs to me that playable Jedi don't have to be an either/or scenario! Force sensitivity can be a graded reward... similar to how Diablo 2 has Legendary items, Unique items, Set items, etc.
In the Star Wars films, not all Jedi have equal power. Also, different Jedi seem to excel at different Force-related skills. So another option for a Star Wars MMO is to make only the most powerful Jedi exceedingly rare while allowing many players some degree of Force sensitivity.
These degrees of Force sensitivity could affect both type and power of Force skills (degrees of power should be reflected in animations, and not just stats).
Also, some players could potentially acquire less-powerful but unique skills. Using the Force to create an illusion, for example, would not be a particularly powerful skill, but most Jedi/Sith would not have access to it and it could be fun.
As I've said many times before, content doesn't have to be experienced directly by a player to improve his or her game experience.
edit: Some commentators over at Massively have pointed out that a Star Wars MMO could also have different Jedi classes and factions. That's not how I would do it, but I agree that it's probably what Bioware's going with if they're basing their game on KOTOR.