Well, first of all, for Lucene, the DocValues mechanism which allows typed storage to be associated with documents has been updated to allow for missing values and there’s now an in-memory supporting DocIDSet which is more efficient for carrying around smaller lists of documents. Other changes can be found in the Lucene 4.5 release notes.
Solr 4.5, as usual, benefits and supports these changes as it is built on Lucene, but the search platform has also had its own set of improvements. For example, when running a sharded cluster, its possible to now set up custom routing to the various shards, including routing based on field values. Faceted searches are now multi-threaded, the solr.xml configuration file is now storable in ZooKeeper and the CloudSolrServer has the ability to send updates directly to shard leaders. Again, more details are available in the Solr 4.5 release notes and the PDF of the updated Solr reference guide is available through the Apache mirrors. Both Lucene and Solr also have various bugfixes and performance improvements.