Pictuo is basically a visual RSS reader at its core. I'm a designer and I was tired of using text oriented readers like Google Reader to consume all my image based feeds. I've been working on this for awhile, but I recently removed the invitation system. Feedback welcome!
I suspect you're referring more to visual design, correct? If you mean design in the broader sense then comparing "Professional" design with "Non-Professional" is like asking if you can test the difference between a product made by someone who knows how to build a product versus someone who doesn't. Anyone building a product is, in fact, designing. Whether they are good at it is another thing entirely. Design is combination of "how things work" and "how things look" (and I'd even argue the latter is still a child of the former).
Visual design as decoration is probably less effective, and I think you'll get a lot of that with "Unprofessional" designers. Visual design as a way to amplify/enhance existing well-designed interactions is definitely effective (i.e. strong button affordance, good use of color to convey meaning, etc). This is the type of output you'll get with a "Professional" designer.
Philosophy aside, I think you'd probably do better with qualitative testing. Do people tend to complete tasks easier if the action being tested "looks" easier? Does the brand identity match the audience you're targeting? Are your design patterns consistent across the site? Etc, etc. It's certainly a bit more nebulous but I think with the right questions you could certainly use A/B testing to answer some of these things.
My general opinion on the matter is that ideas and execution together are valuable. A good idea (validated market?) with poor execution is likely to produce a poor product. Conversely a bad idea (no market?) with outstanding execution is also likely to produce a poor product. Pitting them against each other doesn't make sense. They're two sides of the same coin. One thing is for certain that we can all agree on: a great idea with great execution produces a great product.
As far as sharing ideas go, I tend to only share with people I know. I violently agree that collecting feedback on an idea is immensely valuable, but there are many ways to go about doing so. Posting them into Hacker News is probably the last thing I'd do, for two reasons:
1) Unless I'm building a super niche product targeted at developers/entrepreneurs/designers, then Hacker News is not the place I should be collecting feedback.
2) If anywhere there exists a capable group of "idea executers", it's here. Paranoid? Maybe, but unless I am going to gain extremely relevant/targeting feedback from my core audience (see #1), why take the risk?