I've always been surprised to see the <defaultSearchField> element and <solrQueryParser defaultOperator="OR"/> defined in the schema.xml file since the first time I saw them. They just seem out of place to me since they are more query parser related than schema related. But not only are they misplaced, I feel they shouldn't exist. For query parsers, we already have a "df" parameter that works just fine, and explicit field references. And the default lucene query operator should stay at OR – if a particular query wants different behavior then use q.op or simply use "OR".
<similarity> Seems like something better placed in solrconfig.xml than in the schema.
In my opinion, defaultSearchField and defaultOperator configuration elements should be deprecated in Solr 3.x and removed in Solr 4. And <similarity> should move to solrconfig.xml. I am willing to do it, provided there is consensus on it of course.