While I am not originally from South Dakota, the 11 years I have lived here plus the 6 years I have run websites has taught me a few things about running a web site from our fine state. With that said there are a few topics for sites that one probably shouldn't start while living here.
1. Celebrity Gossip Site - Other than Sturgis Rally week, stalking out Sioux Falls or Rapid City Regional airports for celebrity pics would probably get quite boring.
2. Tech Blog - Kind of hard to write about cool gadgets when most aren't even available here.
3. Traffic Site - Traffic? Other than 41st and Louise, what is traffic?
4. Sports Blog - Somehow I think the Skyforce and Canaries have limited appeal outside of the Sioux Falls/Tea/Harrisburg "metroplex".
5. Gun Control Advocacy Site - Did I say this was South Dakota?
Of course there are exceptions to every rule but this would be a good list to start with.