A lot. Lucene is basically the inverted indexes, providing on-disk structures and a mechanism to query, as well as assorted bits like tokenization.
ES adds distribution (multimaster-replicated cluster of nodes connected via a gossip protocol), sharding, defines a document model and schema (the mapping of arbitrary JSON documents to index structures), faceting, aggregation (ie., roll-up-type calculations), various types of scoring (eg., geographic distance), ETL ("rivers"), backup/restore, performance metrics, a plugin system (eg., for indexing different file formats) and a bunch of other things -- and of course a REST-based API on top of the whole thing.
ES adds distribution (multimaster-replicated cluster of nodes connected via a gossip protocol), sharding, defines a document model and schema (the mapping of arbitrary JSON documents to index structures), faceting, aggregation (ie., roll-up-type calculations), various types of scoring (eg., geographic distance), ETL ("rivers"), backup/restore, performance metrics, a plugin system (eg., for indexing different file formats) and a bunch of other things -- and of course a REST-based API on top of the whole thing.