major technical: no privilege model

There is no security or privilege model.  I suppose this might be construed
that there is a very coarse-grained privilege, either a user can see
a graph (ie, the implementation resolves the graph's IRI for the user)
or not.  And implementations might take that a step further and
arrange things so that one IRI might name a subgraph of another IRI.
If the graph or the user community is large,
the administrator will probably prefer the ability to control access
with fine-grained privileges. For example, the administrator may wish to
grant or deny access to triples on the basis of the predicate, the subject,
the object, or perhaps by reachability from selected starting nodes. 

Fred Zemke