The one thing that has always impressed me about the Star Wars setting is the sheer scale of it. This isn't just a couple warring planets, or even a couple groups of planets, or part of a 'quadrant' of a galaxy. This is an ENTIRE GALAXY, with a 'thousand thousand' worlds (at least discovered worlds). It is difficult for people with a 'planetary' mindset (like us Humans) to comprehend how big that is, though I think the movies do a great job of conveying it. The Death Star shows how 'titanic' a scale things can be in this universe, as do the seemingly endless variety of alien races shown. Even the canon 'history' of the Galaxy, stretching back THOUSANDS of years of spaceflight, gives the feeling of something almost unfathomably huge.
For a game master who runs Star Wars, this leaves the door open for just about anything you can conceive of. I've spoken before on how 'all inclusive' the setting is, but it bears reiterating, because I think it is one of the chief strengths of Star Wars. For example, in an adventure, you could have the Empire wipe out an entire world (as they did with Alderaan). This is a truly horrific act with a lot of dramatic repercussions for roleplay. But from a gaming perspective the destruction of one planet isn't going to upset the balance of the entire game. Its like having your cake and eating it too. All of the drama you want without entirely changing the face of the setting.
The same can be said for introducing a variety of different settings. You want an old-west world? No problem. An Imperial Rome world? No problem. Hell, I even had a world that was based on Transylvania (complete with a dark Force mad scientist and his genetic monster). And all of it works because there are so many 'nooks and crannies' in the Star Wars universe where such oddball settings could exist. In the Marvel Star Wars comics, for instance, they had a whole adventure taking place on a world very much like 'Barsoom' (mars) from the Edgar Rice Burroughs series. And you know what? It didn't even feel out of place.
I bring the issue of 'scope' up because of a couple things. First of all, I've heard some folks say that Star Wars (or any game based on a movie or book series) is too 'restrictive'. That everything is 'written' and characters would have to live in the shadows of the main storyline- ever in fear of upsetting the apple cart. This conception is just plain wrong. At least with the Star Wars setting. If you can't find room for your own stories in THIS setting, well.. then you have a really uncreative GM. If you're a stickler for Canon, perhaps there is some restriction- at least when it comes to developing ultra-powerful Force user types. But other than that? I don't see why there can't be room for a whole lot of 'big damn heroes' in a setting this size.
The second thing I've seen in some GMs is a failure to grasp the scope of the setting, namely when it comes to NPCs and the impact of player actions. While I am a huge fan of allowing players' actions to shape the course of the Galaxy, I think that some GMs forget that there are a LOT of other folks out there who may be obstacles to player's plans. Nobody in the Star Wars universe acts with impunity. Even the Emperor, as powerful as he is, has to constantly scheme and plot to keep what power he’d gained. You also never see him trying to single-handedly hunt down the Rebel alliance himself. Why? Because it is dangerous. Even to a person as powerful as him. I've seen it before, particularly in online RPGs (MUSHes specifically), that a powerful character might get to feeling too big for his (or her) britches. To this I point out that even a high-powered sith lord can be taken down by a horde of lower-power grunts. If you make use of the combined action rules (something I consider a CORE system in D6), this is very true.
For instance, a Sith appears in the midst of a New Republic Controlled Coruscant. He takes hostages and begins making demands. The person playing the Sith thinks, because he's uber powerful, that only HEROIC characters could possibly stand against him. Thus, he prepares for an assault by Jedi or the like. Meanwhile, Coruscant has a huge military and police presence- including elite teams of anti-terrorist commandos and snipers. They could mobilize this group against the Sith. Even though individually they aren't as powerful- and many may be lost trying to take down the Sith, an anti-terrorist team would pose a real threat. This is something that I feel a GM should keep in mind. It is a bit of a balancing act- making sure players feel they are 'powerful', but they should do so with some common sense and discretion, otherwise, the setting devolves into a 'superhero/supervillain' type setup- which is great for a superhero RPG, but... not for Star Wars (in my opinion).
Anyway, I know that this post is rambling all over the place, so I’ll try to wrap things up. The Scope of Star Wars is what really makes it unique as an RPG setting. To my knowledge, there is no other mainstream setting as large as this (with the possible exception of Traveller or maybe Warhammer 40k- but even there, I don’t think the civilization spans the entire galaxy). Even if you DO include all the expanded universe crap in there (and a lot of it IS crap), you should still have plenty enough room to do whatever the hell you want as a GM- or a player (just so long as you realize there are a lot of other folks trying to do what THEY want, too).