A few months back Tom Shields of Woodside Funds wrote a post for VentureBeat on investing principles for web 2.0 companies. Nisan Gabbay of Startup Review subsequently distilled it down into 7 indicators of consumer Internet success. Let me suggest only 5 keys to success.

Five Keys to Web 2.0 Success

1. Solve a problem, and make it easy (flickr - photo sharing, stubhub - ticket sales)
2. Ability to leverage free user acquisition (natural search, viral marketing, widgets)
3. Service that enables commerce (ebay, skype, adsense)
4. Shrink a market (wikipedia, itunes)
5. Compelling story that attracts mass media (youtube, netflix)

I'm sure this list could expand exponentially, but for the most part this seems like the short list. Of course no startup will leverage all of these, but the more they accomplish the higher the likelihood of success.