How are driving the LCD? 8 bit or 4 bit? If you're using up all the lines, it would seem to indicate you are using 8 bit mode.
This board would be more useful (closer to a Handyboard), if it used the 4 bit interface, and then used a shift register for the buttons, to free up more of the pins for interfacing in "stand-alone" mode.
I will say its a great effort for a "first shield", though; clean, compact, double-sided - that took some work designing and laying out, no doubt there. I hope you take what I said above as constructive, though - maybe for version 2.0?
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Hmm - on second look; maybe you already are using the 4-pin interface (with other pins being used to control the light and brightness/contrast, maybe?); its difficult to tell from the pics released. Maybe there needs to be two shift registers - one for the display and one for buttons...?