The "how it works" boxes on the first page need to give me more. I kept clicking on them.
I liked the plans list. I want the entire box to be a click target, and give me a mouse-over subtle color change. I also want pictures there.
The checkout page is very weak. A little javascript to count down how many vegetables I have left and gray things out when I don't have any options would be nice. The current plan upgrade link was sloppy-feeling. I also don't like "Oops! We need a little more info. Could you fix the following things?" ..it made me feel like I screwed up, when I think I followed the obvious path through the application. I'd be tempted to split out the joy of choosing what plants I'm going to get from actually paying for things. Also consider making the username the same as the email address, less to remember.
Come to think of it, after I choose a plan, a page where I romped through (conceptually) a bunch of awesome-looking fruits and veggies, dreaming about my awesome garden, with a form down at the bottom that says "where do you want us to send your tasty, tasty carrots, beets, and celery?". Then the address capture is about where my goodies are going, not paying...
This website is a great idea. I wish I had space to plant things, I'd give you a shot.