A cross-platform java client would be best.
I second that and if done that way I pledge another 10 BTC. A modular architecture would allow users to select what financial instruments / functions (demo-ed on the MoneyChanger) they would like to use and select them during installation phase or install / uninstall modules later. The GUI must be as simple as possible with few big buttons geared towards younger generations instead as a target group. I'm also skeptical that the average nowadays grandma will ever use this product. OT is way ahead of its time! First bitcoin must dismantle current monetary monopoly and inflict changes on monetary regulation and legal frameworks.
I like multibit's take on this, 3 fixed tabs in the app and all others can be closed (if you open them).
IMHO
http://www.voucher-safe.com/ is much better.