Two days (of four) into Elasticsearch Engineer II training, with some background in ES 2.x, and having recently done this video course, the most interesting things are:

• _reindex API
• update_by_query
• Painless scripting
• ingest pipelines
• search templates
• significant terms aggregation (though I’m not sure I exactly understand it yet)
• an alias can point to multiple indexes (or a wildcard)