It doesn't matter if you do all the error checking in the world, or if you have the most beautiful graphics, if your site or application design isn't usable, it's not going to do well. Get input and advice on usability and user interface issues here.
I think the arrow should be a gradient that goes to a lighter green as it goes downward (but still darker than the inner green) and the bottom bar be the same color as the lighter shade of the gradient. ^_^