The most important element in the game is your cast; you can select these from any creatures on your computer, go online to steal somebody else's (and let's face it, before we all abandoned Spore we created an awful variety of creatures) or create something specific and new. Likewise, plants, vehicles, and buildings can all be rejigged to fit in with your plot. Special effects and sounds can be dropped in, and have the same intensity/range sliders as everything else in the game. Then you can terraform your planet, raising and lowering the sea-level, carving out mountains and valleys, and selecting a range of plants to cover it, all to create the perfect backdrop to your plot.
Once you've added an element you can rescale it, create duplicates of different sizes and set default behaviours, paths and teams for creatures and rely on the cleverly primitive AI to handle the rest. You can also add speech bubbles, dialogue and random thoughts, as well as editing all of their vital stats. Everything has the same user-friendly drag-and-drop interface as Spore, and the limits on level complexity are set very, very high.
Now you've got all the materials you can make your adventure. Your captain has to complete all the specified goals (simple things like "talking to Satan" or "befriending the Pig Bride") before an act is complete; once all the acts are done, the mission is over and the player is rewarded with their points. There's a somewhat arbitrary limitation imposed by only allowing you eight acts of three goals each, but that should still be plenty to make everything for the throwaway missions that Maxis seems to want. We think there's definitely the potential to make some interesting vignettes, but not enough elbow room to make a Telltale adventure or match something from the Adventure Game Studio
As people complete missions they get to rate them (a simple thumbs-up or down). This is the only element of the reward system that could be troublesome. As City of Heroes: Mission Architect showed, if you have a game-related reward system for players it provides an alternative rating system for those playing it to the actual quality of the mission - as in, I'll score your mission higher because it gives me rewards for doing nothing. Thankfully, Maxis has made the points system independent of the mission's score, but there's still an incentive to give shorter, easier missions higher scores.
At the time of writing there weren't enough missions to level yourself up very far, though you could make some yourself and grind them. Moreover, if you had decided to make your race deficient in any regard, certain of the goals of the missions might be impossible - most level-creators will assume you've rationally created a do-anything, go-anywhere Captain, rather than the possible legless polka-dotted blob you may have persistently advanced to unlikely galactic domination.
Finally, though the Spore catalog is stuffed with useful creations, we still found it quite slow to load and not very easy to browse. Searching by tags is fine, but the mass of content (which includes a high proportion of mediocrity) means it can be hard and mind-numbing to find the best model for your job.
This game hasn't really been made yet. In its current state there are very few missions and you're going to get bored doing the same ones again, especially as you don't garner any rewards from them a second time through. However, once those millions of monkeys with typewriters start churning user-generated content, and people get to grips with the simple-but-powerful tools, we expect to see a lot more interesting stuff here. Once there are a gazillion X-rated ambulatory penis missions, this will be the natural conclusion of the original Spore game and, as Kieron said also the natural conclusion of its paralleling the history of mainstream videogames; welcome to the Little Big Planet stage.
If this rational extension had been in Spore from launch, it may well have retained the lasting appeal that it obviously lacked. As it was, despite Will Wright's high concept glitz and glamour, the ultimate product was compromised. I've not seen anyone playing Spore on my Steam friends list since two weeks after launch and, with the Sims 3 in the wings, it's hard to be sure this will lure them back. Not until the World of Cocks videos start hitting Youtube, anyway.