Basically, spore is as follows

The cell game is an asteroid shooter with a customizable ship. It's decent, but not as much fun as Geometry wars, or even as much fun as a traditional asteroid shooter.

The creature game is a third person action RPG. You have a character, you fight monsters for Xp, you can recruit a small party, and you can customize your character. It's sort of fun, but again, it's not as good as an actual, full function ARPG would be. The control isn't that good, the 'animals' don't behave convincingly, just sort of milling around their spawn points instead of hunting, nesting, grazing (there are no grazing animals in spore). All animals are pack animals, there are no solitary hunters or herd animals, and they all use the same limited AI.

The tribal game is a squad level tactical game, but again, it's badly lacking in features. You build a tribe of up to a dozen guys, fight a few other tribes, and then your done. Poor control and pathfinding mean that you're basically zerging your guys at their guys until you win.

The civilization game is an RTS, and in keeping with the pattern it's lacking features. You have one resource and six units to choose from. It doesn't matter how you customize the units, because they're just expendable zerg fodder. You have one air, one land, and one sea unit for military and religious bases. Which means you can't really use any tactics because all your troops do the same thing, you don't really get enough of them to have multiple groups, and anyway the game doesn't really reward tactical or strategic thinking.

The space game is any good space empire or space trading game, but without any of the sophistication and complexity that makes those games truly enjoyable.

Basically, spore is five mediocre games strung together in one attractive package. None of the components are good enough to stand on their own. When you look at all five together, instead of a combination that is better than the sum of it's parts, what you get is a clear sense that, at any point in the game, you'd be having a lot more fun playing Asteroids, Diablo, Populous, Civilization IV, and Masters of Orion.

The real shame of it is that Spore is almost really great. If it was just a touch more sophisticated, If the creatures in Critter mode actually hunted, moved in herds, stalked, mated, and so on. If the diplomacy in tribal mode actually mattered, if you could lay real ambushes, if hunting or farming were important instead of just time consuming. If Civ mode actually allowed you a variety of units, some semblance of combined arms, some semblance of tactics. If the space mode gave you better control over your civilization, instead of leaving you to micromanage a huge and complex area stretched over literally thousands of star system, Spore would have been great. And it's not. It's just five mediocre games.