Puzzles provide relief from combat, and vice versa. And the friendly NPCs are almost never fair game. The guards are mean. Something neat, something that gets the imagination going. Like, for example, the fact that you can buy your own house in Exile III, and there are tiny adventures that go on inside the house.
People absolutely love that.
Another example is the realism and detail in the Ultima VII world. I loved the way you could search through the trash you find, and find cool stuff in people's dressers. That was great. My next game is Nethergate, a historical fantasy RPG set in Roman Britain. I have a lot of great ideas for this, including your being able to play a Roman party or a Celtic party.