I'll admit, I think one of the thickest barriers to entry regarding Linux is variety. I love variety, but general end users don't adjust well to change (IMHO). Thus enter the stage: Portland. So, what does this mean to the end user? This means that, eventually, the UI throughout linux will all look similar -- regardless if it is designed to utilize GTK, QT or anything else (that is, if it is built off the Portland standard).
And as a runner-up, worth mentioning, the user driven Tango Desktop Project