Sorry, but for me its the rules. If you don't have an established rules mechanic everything else breaks down and become rather inane. Look at the challenge level of KotOR1 for an example of that. Think how much tougher the game would be if they did the following:
1. Use the Wound/Vitality System rules for combat and force use.
2. Made full use of combat rules such as trip attacks and attacks of opportunity.
3. Proper damage ratings for blasters, rifles, and lightsabers.
4. More detailed skill system, such as breaking down Persuade into Bluff (for Scoundrels/Consulors), Intimidate (for soldiers/guardians), and Diplomacy (for scouts/Sentinels).
5. Being able to shoot targets kneeling or at prone.
There are more but these are the most important in my opinion.
d20 System isn't DnD, but DnD is a derivitive of the d20 System. The DnD rules system is consistant to the DnD rules system. Star Wars d20 is a derivitive of the d20 System but if you look at all the Star Wars d20 books they all use the Star Wars d20 rules pretty consistantly. The same can be said with Babylon 5 d20. It is a derivitive of the d20 System but every book that has the Babylon 5 d20 logo is consistant with the Babylon d20 rules. See the pattern here?