<p>As an example, Google App Engine uses Google Datastore, not a standard database, to store data.  Does anybody have any tips for using Google Datastore instead of databases?  It seems I've trained my mind to think 100% in object relationships that map directly to table structures, and now it's hard to see anything differently.  I can understand some of the benefits of Google Datastore (e.g. performance and the ability to distribute data), but some good database functionality is sacrificed (e.g. joins).</p>

<p>Does anybody who has worked with Google Datastore or BigTable have any good advice to working with them?</p>

Being rooted in the database world, a data store to me would be a giant table (hence the name "bigtable").  BigTable is a bad example though because it does a lot of other things that a typical database might not do, and yet it is still a database.  Chances are unless you know you need to build something like Google's "bigtable", you will probably be fine with a standard database.  They need that because they are handling insane amounts of data and systems together, and no commercially available system can really do the job the exact way they can demonstrate that they need the job to be done.
(bigtable reference:
http://en.wikipedia.org/wiki/BigTable
)