1. Geography: Atlanta is sprawled out, so even if you live near Tech or Emory, odds are you can't skip down to the local coffee shop to get a feel what other startups are doing.
2. The Business Mix: Silicon Valley is tilted heavily toward technology in general and therefore also to startups (at least the kind we're talking about here). In Atlanta your friends and neighbors are likely to be bankers, accountants, healthcare professionals, or any number of corporate types than startup founders or even early-to-medium stage technology employees.
3. The Business Culture: There's little "porosity" among companies -- the knowledge/social network isn't there, and that's probably the most important ingredient of a startup hub, more important than money, IMO.
Interestingly, I think this might be an scenario where a YC clone could do the most good -- because as we are always hearing here, its real value is as an aggregator of like-minded people, enabling some of the knowledge interchange and social connections that don't happen naturally.
2. Great point. Real estate is king in ATL. Read "The Man in Full" for more.
3. I think the social network is here. I literally sit and look at over it every day from my office window.
Atlanta doesn't get many Christopher Klaus's - and they typically leave, while the list goes on and on in valley... the culture in Atlanta is just too different.
Just head down to Octane one day. I can guarantee that at least one person there is working on a startup.