This is another subject I've already touched on in other posts, so I won't belabor it here. Enjoyment of gaming depends upon personal preference—from the genre(s) you most like, to the character types you like right down to the rule systems. This is one of the reasons I hate it when people say one system is better than another. Yes, I do champion the D6 system because I like it—but that doesn't mean I think it is the pinnacle of game design or that it is better than D20 for reasons x, y and z. I just like it more than other systems—that's my personal preference.
That being said, one of the main reasons I DO like D6 is its skill-based system. This means that characters are defined according to what skills they have, rather than what 'class' they belong to (as they are in many traditional game systems). In classic games like D&D, characters were rather broadly categorized (at the time I got into the game) into four main areas: Fighter, Thief, Magic-User and Cleric. A couple specialties evolved from each of the basic classes, but that was pretty much it. Apart from combat and special abilities, most character classes didn't even HAVE skills associated with them. Thus, if you were a fighter, that was what you did. There was no 'hunting' skill or 'blacksmith' skill or anything like that. Only the thief had easily identifiable skills: move silently, open locks, detect noise, etc.. Thus, with the earliest class-based system, everything seemed geared towards how a character fared against typical dungeon obstacles: their combat skills and spells to defeat monsters or the thief's abilities to overcome traps and locks. Yes, ALL characters could search for secret doors and the like, but that 'skill' never changed. A 1st level character had just as much skill at finding secret doors as a 10th.
While I appreciate the simplicity of this Class mechanic, even from my earliest days of gaming, I remember it not sitting right with me. Again, I point out that this is my preference, not a shortcoming of the game—as a kid, I had a lot of fun with D&D, using it pretty much as-written. What really opened my eyes to other possibilities was the Star Frontiers system. This TSR game had something of a hybrid between classes and skills. There were three primary character types: Military (soldiers), Technician (techs) and Biosocial (medics/scientists). Each of these 'classes' had certain skills associated with them (weapon skills for soldiers, tech skills for techs, etc.). HOWEVER, any character could purchase skills from any 'class'. Thus, you could have a tech with weapon skills or a medic who could use computers. It cost a few more experience points to purchase skills out of your primary area, but it was possible. This I liked a lot, even if I eventually found the small number of skills in the game somewhat limiting.
I dabbled in other skill-based systems in my gaming (Top Secret/SI for one) but it wasn't until 1987 and the release of the Star Wars RPG that I found one that I really loved. With this system, you could build whatever kind of character you wanted to. You weren't limited at all by any one particular focus. You could have a thief who could fight, a tech who was also a Force user or even a jack-of-all-trades. The sky was the limit. And if your tastes for a character changed, you could even take an established character and (with experience) change them into something new by spending points on different skills. Everyone in my campaign was like this to some extent or another. Arianne was a Pilot and a Jedi and a Spy—mixing skills from all those categories. Harry Hugganut started off as a street-punk, with thiefy and con-man type skills, but quickly became a gunman and a gambler and then turned his interest towards medicine. Adren was a smuggler and a Jedi and a swoop racer. Oman was a Bounty-Hunter who wound up adding some political skills into the mix when he became ruler of Mandalore.
To me, this 'lack of definition' of class was liberating. It felt more 'real' to me. Just as in real-life I would not define myself by one thing. I am not a '7th level art director' or '8th level gamer' or 20th level star wars fan'. I am all those things and more.
The latest iteration of the d20 system tries to address some of this, both by incorporating a skill system and by allowing multi-classing. But to me, it feels awkward. If you're going to allow multi-classing and crossover skills and all that, why would you even NEED a Class system? It just seems like a holdover used for almost nostalgic reasons (i.e. people expect classes in a D&D system, so we're going to give them classes). I would prefer they chose one system or the other, rather than trying to do both. It's like Mr. Miyagi said (and I'm paraphrasing): You skill system do yes, is good. You skill system do no, is good. You skill system do 'hybrid'? SQUISH, just like grape.
So in closing, I would like to continue with just another highly subjective thing: I love Jacks-of-all-trades. I like characters who have something to contribute in a wide variety of areas. My own characters are like this and I appreciate it when players in my campaign broaden their horizons, too. It's a good way to make sure that no matter what the situation, odds are your guy is going to have something to do, even if it is just supporting the others in the party. Star Wars is the ultimate example of this—everyone can at least ATTEMPT pretty much any of the 'basic' skills out there. I have to look no further than my own campaign for a good example of this—when Bob the Tusken Jedi suddenly found himself the only person able to land the ship he was on (I forget what drastic reasons there were for this, but it did happen). As he positioned himself behind the controls, he uttered his famous quote: "How hard can it be to land ship. Planet large. Ship small." True dat, Bob. True dat.