Are there any sites or services where someone can describe what they're building and how they plan to do it (e.g. web plus iphone apps using AWS or docker, imgix, cloudant, bootstrap, mapbox, etc) and either locate an expert to review design decisions/make recommendations, or have "the crowd" review it? A little like 99designs but for the backend/architecture part.
You should try <a href="http://www.quora.com/" rel="nofollow">http://www.quora.com/</a> . There you can even ask to people you consider expert. About the undecided things, This will depend also where live, like you can choose Node.js but there isn't (almost) nobody that that works with that near you, then you would need to hire someone from another state/country. RoR would be pretty good for the factor it is already well known and if your case yours requirements is the speed to create a product. Node.js and others (like Java, Scala, etc) you could get a better performance, but it could cost more and time more time. I would not recommend hybrid mobile plataform such phonegap or HTML5, unless the time contracts is short and the user requirements are very low. You can use <a href="http://phabricator.org/" rel="nofollow">http://phabricator.org/</a> as bug tracking and sonarqube (like <a href="http://nemo.sonarqube.org/" rel="nofollow">http://nemo.sonarqube.org/</a> ) to QA over the code.
In other words, is there a forum or service where you can list high level requirements, list frameworks/libraries/technologies you plan to use, and have people critique that or suggest alternatives. And in case it wasn't clear, I'm absolutely fine with this being a commercial service, whether it's an individual freelance or more crowdish like 99designs.
I know that some enterprise put this kind information in their engineering blog (for example the Linkedin: <a href="http://engineering.linkedin.com/" rel="nofollow">http://engineering.linkedin.com/</a> ). But What more exactly your are thinking to use?