Loved the use of theme in both goal (Billy) and mechanic (plants). Very clever!
I thought it had a ton of replayability, as I found myself convinced that I could do just a *bit* better with one more try.
The display was a little hard for me to read -- I would have preferred larger type and a bit more color in the countdown, etc -- but that's the kind of polish that normally takes more than two days to implement. : )
Also loved the speed multiplier -- I thought it was a great way to let players get their head around gameplay.
Very well done!