Finally, yes -- there is no theoretical expressivity gains between RDBMS and property graphs (and, RDF graphs). Nor is SQL (Turing Complete versions) any less expressive than Gremlin (Turing Complete path recognition). The only argument you can make is that graphs are more (or less) effective in terms of conciseness of expression and speed of execution at particular problems. Typically (as expected), its the difference between problem datasets that look like networks (graphs) and those that look like spreadsheets (tables).
Furthermore, please have a look at "On Graph Computing" for a break down of 3 different categories of graph computing systems -- toolkit, database, analytics. http://markorodriguez.com/2013/01/09/on-graph-computing/
Finally, yes -- there is no theoretical expressivity gains between RDBMS and property graphs (and, RDF graphs). Nor is SQL (Turing Complete versions) any less expressive than Gremlin (Turing Complete path recognition). The only argument you can make is that graphs are more (or less) effective in terms of conciseness of expression and speed of execution at particular problems. Typically (as expected), its the difference between problem datasets that look like networks (graphs) and those that look like spreadsheets (tables).