I think a few other people have touched upon this, but my summation of what Spore will have to offer is that you shouldn't think of any one stage as a full game, with sufficient depth, etc. The depth of Spore will be that you go through all these stages (tidal pool, ocean, land hunting, tribes, etc) and they build upon eachother, the things you do in these stages may seem simple, but they have a lasting impact on every stage afterwards.
Example: (* refers to things where I can't be absolutely certain, from what we know so far, that such a feature exists)
IE, your creature shambles out of the water and begins devouring smaller creatures for strength and evading more powerful predators. In so doing, it wanders deep into a desert landscape where perhaps prey becomes more scarce and it must develop sharper senses* or the ability to move farther or faster, and needs much less water, perhaps it develops special camouflage* to hide from giant saurians that prowl the wastes?
So skip ahead, skip ahead; those desert people who evolved in their environment purely by chance form their entire society and technology around surviving and flourishing in this environment, OR, perhaps you grow sick and tired of eking out and existence int he desert, and you now have a possibly difficult quest where you move your entire tribe to someplace where the environment suits your wishes.
You can probably see what I'm talking about here, that choices and happenings in an earlier stage would very logically have an impact on every stage thereafter, and without necessarily needing a great deal of depth in any one stage. Also I've pointed out that, like Will Wright's previous games, it is the gamer that determines what they want to get out of the game, it is the gamer that choses their goals. But now instead of playing the Sims 2 and trying to, say, make a 'graveyard' lot just for the freaking hell of it.. you get choices like making vicious desert-wandering Carebears with energy rifles that drive gigantic tanks around (Jawas!) just for the freaking hell of it!