Requiring a user to type in a zip code for the registration is awkward when all that is required is a country (and zip codes only apply to the united states).
I like how it find matches for products that have already been entered when you submit a new one, but it seems that only the first submission will set the product's URL and description, which is what most people will see when they view an item.
Also, it's not obvious at first what URL should be submitted for a product. I wanted to add 'Blast Corps.', but still am not sure what should be done for a product no longer in retail.
It looks fairly well done. I prefer the random bag method of piece generation. Also, the movement of falling pieces is a little touchy--sideways movement can sometimes generate a double movement and sometimes no movement.
However, if you play around with the game on meatbrick.com a bit longer, you'll see there is more to it than a Tetris clone.
Kongregate fundamentally isn't in game development--it's simply another form of distributing UGC. I'd rather be in the business of creating great games.
Thanks for the comment on the color scheme. I've altered it slightly to be less headache-inducing.