Over on Elder Geek, you'll discover a retrospective piece on Star Wars: KotOR, BioWare's original D20-based RPG that sparked enough interest to prod the developer into taking two separate development paths - the Mass Effect trilogy and a highly ambitious MMO. Unfortunately, I'm not sure they've replicated the same brilliance that Knights of the Old Republic exhibited so many years ago:
Like I said before, KotOR is based on the d20 Modern system. This means that your damage, critical change, persuasion, and just about everything else is governed by a virtual dice roll. However, instead of making you learn all of those rules they boot you right into the game and let you fend for yourself. Thankfully they make it easy to figure out which weapon does more damage and what armor is better suited for your character, because half of this game is about killing everything that crosses you and looting their bodies.
The other half is about talking to people and making the now infamous moral choices. There are a lot of choices you have to make and most of them are either super nice or super nasty. For example, you can give money to someone to help them buy medical supplies, or you can just kill them on the spot because you can. As you make choices your morality meter goes towards the light side or the dark side and you get boosts in certain force powers depending on how much of a dick you feel like being that day.