Something that the HackNY speaker series had that DemoDays hasn't really, is speakers whose primary purpose is to present special knowledge. I'm not sure if this is appropriate or desirable for DemoDays, but it's probably useful to point out.
Joel Spolsky, Chris Poole, and Josh Knowles, etc. were these awesome people experience about creating successful products and developing as a promising young hacker person.
Other speakers had interesting and related knowledge to impart that we may not have necessarily had. I missed her talk, but heard Whitney Hess talking about UX was awesome. USV offered interesting insight into the world of VCs. Jenny Wortham talking about how to build a relationship with the press was incredibly useful.