One of the best reasons to take a European River Cruise or any river cruise for that matter is the immersive experience you get. Unlike an Ocean cruise where most of your time is spent onboard enjoying the ship, on a river your time is spent enjoying the destinations. This come in the 2 parts of your journey.
The first is when your ship docks in the center of the town and you get off right in the middle of the action you get a true breath of the town. You see it as it has been seen by visitors for all of history. Since most of these towns grew from the river banks the historical parts of the city open up for you. And since these boats are small they often port at smaller towns and villages where the locals great you with celebrations and stories about there history and culture. In most of the ports we simply walked off the boat and into our walking/hiking excursions. We explored the towns, the shops, the castles, the Cathedrals, the eateries, the vineyards and then returned to our floating luxury hotel to take in the view as we floated to our next exploration.
The second is once you are back on board and leave port you get to see the country. On an Ocean cruise you end up with long tines at sea between ports with nothing but water to look at. On a River you get everything to look at. You see small towns, villages and people living their lives, as your river cruise winds and bends through the heart of fascinating countries providing a panoramic view of the Old World. Always within a few feet of shore, your river vessel allows you to float past castles, vineyards, villages, and capital cities. Every day and night you have quick and easy access views of life. 
On our last river cruise we voyaged the Rhine Gorge and saw castle after magnificent castle. 40 Castles in all along this beautiful stretch of river. Along the way our cruise director told us about each as we sailed by.
This combination of experiences is something you can only get with a river cruise. The ease of it all was amazing and I recommend it to everyone. Come experience the world.