To be successful and potentially up-end Facebook, Google+ must be adopted by the masses; those who check in on Facebook five times a day and who carry on important, personal, and farcical conversations. Activities that Facebook members thrive on should be replicated and improved upon. Farmville had a transformative impact on Facebook's fortunes. Google+ will need a game to draw members back to the service on a daily basis; Google's hesitancy to bake search into every aspect (it is in some areas) of the service undermines Google+'s most critical advantage.
There's a lot of heat around Google+ right now (you can thank me later, Google, for not saying "Buzz"), but it's when that heat and flurry of initial activity from first movers and early adopters fades that we'll learn if Google+ has what it takes to become the next Facebook.