As a student of both biology* and game design, I have followed Spore with great interest. Since my copy arrived in the mail, I have spent most of my free time playing the game.
Spore is a technological breakthrough dressed up in cutesy visual design. The creature eyes, sounds, and procedurally generated animations are endearing, but there's a less-obvious source of charm: every creature's torso, tail, and limb segment is nearly circular in cross-section.
You can shrink a Spore creatures' eyes as small as they go in order to obscure their adorableness, but there's nothing you can do about the rounded bodies.
It made me a little sad to discover that you cannot laterally or horizontally squash and stretch body segments. I tried using Shift-Mousewheel and other key combinations in an attempt to discover hidden creature-shaping features, but I didn't have any luck.
So, ultimately, there is no way to flatten your platypus's tail. You cannot make a disc-like body for your lizard, a deep torso for your horse, or a broad chest for your ape.
No segment is allowed to shrink below the built-in minimum thickness, either. There is no way to make a gracile leg for an insect or a bird, nor can you properly taper a tail.
All in all, these limits don't detract from the enjoyability of the creature creator. While they enforce a certain humorously cute body type, that type can take many forms.
However, no number of spikes, claws and toothy jaws seem able to make Spore creatures less cuddlesome.
* My best college paper incorporated kinglet banding capture data from Manomet. I wanted to see if evolutionary pressure could be seen in action on kinglet populations; would the size of birds caught be smaller in warm years (since being small allows them to feed more effectively at branch tips), and larger in cold years (since being large allows them to survive cold weather)? The data, sadly, were inconclusive.