Like others have said spore had a great concept it just felt so...limiting after a while. For several reasons in my opinion such as,
1)The Cell Stage was a good introduction to the game but it could have been much more in depth. Especially when it came to creating your cell and your interaction with other cells.
2)The Creature Stage is a prime example of what I think the whole game suffered from,a huge world with little to do in. I mean it's fun for a while and some moments like when your ambushed by a random hostile species, or when you battle an Epic, or when you go on a killing spree across everything you see with a couple of (Rogue?) creatures.
These were when the game shined to me, but after a while you realized there was no point to this because you would just advance to the next stage and that when you reach a certain point in a stage that that stage is over and you can't continue the stage to it's full potential.
3)The Tribal and Civilization stages, to me, were the most well done but really could have used more detail and diversity.
4)Then The Space Stage the biggest disappointment to me because in the beginning it was great. First contact, interaction with other species, the upgrades for your ship, and the great interstellar wars. However after a point it gets ridiculously frustrating when, for example, your exploring on the other side of the galaxy and your empire is falling apart from pirate raids, ecological disasters, Grox invasions, and the like, because apparently your the only person in your entire empire that can do anything. I mean conquering the galaxy or defeating the Grox is impossible when your entire empire is falling apart around you. If you could build other military ships and soldiers for invasions and defense and policing your territory the Space Stage, in my mind, would be 300 times better.
5)Finally the game was just so terrible after a while because it teases you all throughout the game of what it could have been and how much better it could be.
That's just my opinion on it. Though in Spore's defense I still play it every now and again, because despite all it short comings it's still a fun game in my twisted Hobotic mind.