I was just logging in to ask the same question. Is there any difference as far as the website owners are concerned? All they can know, unless they're looking for patterns generated by automated scripts, is their site is down (or just very unresponsive.)
What would the difference be between a truly evil DDoS of www.mastercard.com, and someone posting a 'get $100 free from MC' link on facebook.com? As far as the network and servers are concerned, NOTHING, their admins can only know that they're at 100% saturation.
Try hitting www.irs.gov after 10pm on April 15th. Should everyone trying to do their taxes during the last hour be charged with participating a DDoS attack?
So, what IS the difference between the Slashdot effect and a DDoS?