Ultimately the success or failure of Bitcoin will be the success or failure to get major retailers to take it, particularly online retailers.
Retailers are a bonus, but Bitcoin doesn't need them to succeed. The remittances market alone is worth $300 Billion USD per year. Then there is: online gambling, bitcoin-as-a-financial-instrument, supply chain management, MMOGs, cloud storage, commercial Tor nodes, and many other uses we can't even imagine right now...