Doesn't need to be a strict child when using a graph (not to confuse with hierarchical) database. Not sure what you mean by dangling edges, at least the Neo4j graphdb guarantees that edges are always connected at both ends. It's also ACID compliant so rollbacks are supported. Sure, tanstaafl, but when dealing with data in a graph structure there's much better tools than RDBMS nowadays!
To answer your question I think things will settle down -- in the long run it's too hard for developers to deal with all the alternatives and their differences.
To get a high-level overview of the NOSQL space you could read this blog entry: http://blogs.neotechnology.com/emil/2009/11/nosql-scaling-to... As Ben Scofield puts it (cited in that post): "NoSQL DBs often provide better substrates for modeling business domains". I think this aspect is often forgotten in the debate. So I'd say: start from your business domain, what are the characteristics of it? Then look for a DBMS that is a good fit. And to get down to the details of some of the NOSQL systems, here's a walk through: http://www.vineetgupta.com/2010/01/nosql-databases-part-1-la...
