This post-mortem by Soren Johnson is still the best explanation I know of the game's failure. It was a mixing of great ideas, that never achieved to work together. Too many compromises made, and not enough cohesion in the team.
In the end, the Cell stage might be the best part of the game. It's successful in what it attempts, and close to the original concept (Power of Ten + procedural content). Unfortunately it's only a mini-game.
(well, Spore can be seen as a series of 5 mini-games actually)
Funnily enough, I kind of found what I sought in Spore in Minecraft. Completely different concept, but in the end the game is about creation affecting gameplay, and reciprocally, and also universes created both procedurally and by players… I mention it because I coincidentally heard about the game in a Spore community forum.

And looking at it now, it's unsurprising that most of its members really enjoyed the game.
Now maybe I'm looking forward to No Man's Sky… Sure it's not as ambitious as Spore was (especially when it will likely be a contemplative game, but I'm fine with that), but it has potential in the idea of exploring procedural worlds. Spore planets were repetitive (and ugly, mostly for technical reasons), whereas the few we have seen of No Man's Sky seems more varied. We'll see, but I would enjoy a game simply about visiting new and unpredictable worlds.