In a nutshell, the hostile response centers around the DRM (Digital Rights Management) mechanisms built into the framework of the game, which means that users can only sign up for one account per serial key and that users can only activate the game on a maximum of three PCs. After three activations users are denied further activations and have to phone up EA to beg for another activation.
[Editors note: as of September 19, EA has announced that it has expanded the installation limit to five machines. Additionally, EA is moving forward with a system that will enable users to deauthorize PCs and move Spore to a different computer without calling EA.]
The attack on Spore in the review pages on Amazon.com was swift and savage. A game that had been widely talked about for two years plummeted to a single star rating within hours of release.

Recently it had 2,713 reviews on Amazon.com, out of which 2,368 give the game the lowest rating possible. Spore Galaxy Edition is also suffering from negative feedback, though nowhere near as many as the standard edition of the game.
The complaints all revolve around the same issues restrictive DRM, restrictive policy on accounts (even the manual mistakenly told users that they could have multiple accounts for a single CD key) and the feeling that for $50 you're really just renting the game until the DRM decides that you've had your fifty bucks worth and locks you out of the game.
To be honest though, sinking Spore because of DRM isnt really fair. Plenty of games prior to Spore have had restrictive DRM models (BioShock springs to mind) and I feel that Will Wrights Spore bore the brunt of years of gamer frustration.
Certainly, after reading most of the user comments I get the feeling that users are venting over more than just the DRM on Spore. As a gamer myself, I've grown tired of DRM on games and run into several difficulties in getting games to run properly that were down due to the DRM, in particular issues such as system instabilities and even not being able to play the game at all. (I've always managed to fix these issues, but time fussing over problems is lost game time.) I still buy games, but not as many as I used to.
Now, the idea behind DRM is to stop people pirating the game, but many people mistakenly believe that this mechanism is in place to prevent widescale piracy. It isnt. In fact, despite being kitted out with draconian DRM, Spore was leaked onto the Internet several days before the official launch, complete with a way around the DRM.
Given that, youd think that EA would label DRM an epic failure and remove it. Thats not going to happen, and the reason is that the DRM is there to prevent average users (those who are law-abiding, honest or who dont know that you can get pretty much anything thats in digital form for free if you know where to look) from being able to casually pirate the game.