I thought "you need to narrow your scope" was a useful suggestion. I guess you think differently.
"You need to narrow your scope. I personally would be more interested in X and Y, but not Z, J, or N."
What you did was give a vague criticism without offering a concrete solution in response. Those kinds of criticisms are always difficult to respond to primarily because, should he follow your suggestion and it fail, you can simply say:
"Well I just said narrow your scope, I never said narrow it to those failed topics."
"Why did you even do this? It's really unfocused. This needs to be more focused if you ever hope in seeing something happen. We need to see more action and less wishful thinking."
"Interesting! This has a lot of potential. I'd love to see topics X, Y, and Z, although that's just me. Send me an email and I can introduce you to some people who might be interested in helping to organize this."