 Please review my DropZap web demo.  It's written using GWT.
drhodes: For me, at first, the color and size had distinct significance.  Instead of changing the size and color of the blocks, how about using one color and tuning down it's saturation, or transparency and keep the size the same. Or just change the size?  Did you try this at first, was it too bland visually?  Also, I thought it would be better too not have a row feed of blocks after every drop. Doing this would more clearly expose the mechanics.  Presumably this wasn't feasible because trivial games could be played by stacking blocks in only one column. This expolit might be eschewed by introducing indestructible blocks, and may also allow arbitrary height of the board.  I think the chain reaction effect is visually appealing, but takes away from game play.  Past the second round of block removal, it's impossible (heh, well for this mortal) to definitively predict the consequences of a particular drop. Badda-bing-bang-bong and half the screen is cleared.  When it doesn't work out so well the responsibility for failure can easily be ascribed to unfortunate randomness introduced by the row feed.  It's a neat idea though, I hope it works out and that in 5 years I'm eating my words :]