"Having classes with some unique atributes, but some that cross-over between classes too. I also like the idea of an "advancement tree", like in borderlands, where you unlock some things before others, and can choose to either be a strong specialist or a jack-of-all-trades-master-of-none kind of player. Anything which rewards you for doing what you're supposed to (not necessarily just killing zeds, perhaps building shelters for homeless survivors and other such acts could net you some points)"