Spore has a lot of potential. if they would have releases an expansion that dealt with the weak points it could have been one of the greatest games of the decade. 
Too bad EA lacks vision.
		
		
	 
This. 
Originally in the game, you were able to move freely from water to ground and back. EA wanted them to remove that so that the game wouldn't appear too complex or confusing. You can watch it in videos on youtube. It was in fact in the game. 
But back to your point, they could have added complexity in expansions. Instead we got a 'cute creatures pack'. They also forced the evolution part of the game to make it so you could have every creature part. There was no reason not to do this, and only reason to do it. So every creature ended up being a frankenstein. The game needed limitations and simulated realism. Instead they went cutesy with it.
The mechanics of the game involved either destroying others, or go and shake maracas at your foe.  (or you go for the neutral route, which just meant you did a little bit of both)  There was no protecting of your nest eggs, they put that migration mechanic into the game to pretend to add complexity. I can't pretend to know what they should have done with the creature stage, but they really didn't do anything. You just walk around dancing or killing until you evolve. 
If they would have cut the cutesy stuff and went with gritty realism, added some good game mechanics on top of a replaced evolution system, then fired out some nice expansions they would have had a hit. 
Hmm, now that I think about it, I vaguely remember a game in the works that was suppose to be a remake of what Spore was suppose to be. I wonder what became of it.