The entire concept of programming language fits right in with modeling games.

class D20PC
{
   int level
   string charname
   string playername
   EnumClassName mainclass
   EnumClassName multiclass
   CBackpack *inventory
   Array *feats
   CSkillList *skills
   Image *charPortrait
   int Strength
   int Dexterity
   etc...
}

It all just flows naturally. I mean if you wanted to build a library where you import game rules and the language builds class templates for you... but other than that I don't think there's anything specific about one language feature or another that's more important than the rest.

Obviously, being Object-Oriented is good. Having access to Databases is good too, but outside of that, any OO language would work. I've done games in C#, C++, Objective-C, and Java, and I don't find that they really lack anything.

[EDIT]

And IMO, LISP would be a horrible choice for programming an RPG game engine. I just have no idea how it would be done.