The line between "casual" and "not replayable"

So, I've been working hard on Zombies vs. Knights the past few weeks, and even though it's my second project, it's been an educational experience.

A question I've been asking myself - with varying levels of success - is at what point a "casual" game is or is not "replayable."  This was one mistake I made in my first project, SHMUP: Orbital Combat, and one that I'm attempting to solve in Zombies vs. Knights.

I've come up with a few answers - some are easy to the point of being self-explanatory.  Others are the most esoteric parts of game design that ludologists struggle with on a day-to-day basis.  In any case, here's what I have:

Feedback - Merely telling your player, "You Won!" or "You Lost..." can have a powerful impact, from a psychological level.  It's a form of authoritative feedback that defines the results of the experience and allows the mind the ability to evaluate a set of choices...and all a game is is a set of choices.

Compelling Decisions - A series of compelling decisions is one thing that will keep gamers coming back to a title for years to come.  Why is Final Fantasy VII a revered title?  It's not Sephiroth's sword, that much is certain.  It's the Materia system - you can set a character up any way necessary to overcome challenges posed by the game.  Moreover, you have to change your setup periodically - for example, certain bosses have different strengths and weaknesses - and also to suit your challenges - Yuffie stealing all your materia comes to mind.

Rewards - For the single-player, short projects I make, this is the hardest thing to get right, from a balance perspective...yet for replayability, it's absolutely necessary.  When a player succeeds at something - be it destroying a wave of zombies in the middle of a match, or winning outright - the player needs to be given something that helps them continue doing so, not only for functional reasons of keeping the player "on top" while the game makes things tougher, but also as a form of positive reinforcement.

My big beef against rewards in single player, casual games is this: you can ruin your balance easily, making what starts off as a real challenge filled with compelling goals turn into a crapshoot for your players.  While it is true that no one has ever gotten griped out for a game that's too easy, it's equally true that a game that lacks challenge and depth can repel players.  Thus, I'm struggling for a means of integrating good rewards into the existing choices and feedback my games have evolved.

I'm sure there's more to a replayable, "addictive" game than this.  Of course, I could be wrong.  The only way to find out is make more games, and test the theories!