I don't think the GUI is the key. Monero appeals most to extremely intelligent people who also have lots of money (aminorex, ArticMine). This type of guys will be the deciders of the price in the short (1 year) term. The masses come later.
It does help some investors who aren't necessarily comfortable with command line tools and don't want to keep their coins on a web site. Unfortunately even the third party GUI wallets at present require some facility with the command line to install and configure.
Only a basic GUI is needed for this target market, not something that would attract or serve the masses.