Minor Description
One thing that causes problems for me (and i assume others) is that Solr is schema-strict in that unknown fields cause solr to throw exceptions and there is no way to relax this constraint. this can cause all sorts of serious problems if you have automated feeding applications that do things like SELECT * FROM table1 or where you want to add other fields to the document for processing purposes before sending them to solr but don't want to deal with 'cleanup'