Types of Fun
Here's a list of types of fun. While the list isn't exhaustive, it does cover a number of the bases. Which of these forms of fun does your game use? Could your game benefit from drawing upon other ways of having fun as well?
Being part of a squad
Aiming, targeting, shooting
Mixing and matching: spells, weapons, defenses, fighting moves, choice of spirits you can summon, attack strategies, etc.
Complicated attacks that include multi-step attacks
 An example of a multi-step attack would be that you (1) leave a rifle on the ground, (2) steal around the corner, (3) see the shadow thrown by an enemy picking up the rifle, and (4) shoot and blow up a gas canister near the corner, which in turn kills the enemy.
Multiple ways of accomplishing the same task
Turn-based moves with an opponent
Simultaneous moves against an opponent
Capturing an enemy
Using scripted sequences to change mission direction and add surprising twists, such as being attacked unexpectedly
Sneaking and hiding, including camouflage, stealing hard-to-procure items, and code-breaking
Other forms of combat-related gameplay, including making and breaking alliances, negotiations and betrayal, bluffing, or commanding a number of NPCs.
Different forms of travel:
Driving (in different kinds of vehicles)
Flying (in different kinds of vehicles)
Traveling over or under the water (in different kinds of vehicles)
Kinetic thrills and competitions, including racing, stunts, and other kinds of kinetic excitement (as in the Spider-Man game)
Being God building and managing cities, armies, ecologies, including resource management
Exploration and discovery
The preceding, in a visually compelling environment, whether pleasant, strange, or frightening
Collecting and putting together sets of items
Torturing (what you do with your Sims is your business)
Asymmetric powers (when different roles have different powers and abilities)
Building a network (like in Tic Tac Toe)
Taboo thrills running over pedestrians, barbecuing French poodles, and such
 Don't dare write me a nasty email on this one if you ever laughed at Something About Mary, any of the National Lampoon Vacation films, any of the Scary Movie films, or about a hundred other films of that ilk.
Building machines (including cars)
Humor (many different types)
Changing the actual landscape or buildings in the game you're playing
Training an NPC to do what you tell it
Keeping pets, worshipers, or other NPCs alive.
Dice, cards, etc. to create semi-randomly generated actions
Choosing what side to play (good or evil, for example)
Mini-games (games within a game) of all kinds
Various forms of online games