I will mention one thing that adds a LOT to my trips - onboard enrichment speakers. I really enjoy having someone onboard who is an expert in the area we are about to visit.
My one river voyage between Moscow and St Petersburg we had the MOST excellent Russian experts ever (Viking River) including 4 guides and one professor from the Moscow U. who specialized in economics. Her insights into why Capitalism has not really taken root in Russia were outstanding.
When you are in Europe, the Med... anyplce with real history, it is good to have guides when you are at the sites you want to see, but it is also a drawback when a guide starts to speak on and on about history, etc when you want to see as much of the sight as possible.
Therefore, I really appreciate a ship that has experts onboard who can give us history, etc, before we reach our destination. That way we can concentrate on the experience of being at the site we came to see instead of standing there listening to a tour guide talk about things we are not seeing that happened in the past (if that makes sense).