Others have answered 1-3, so I'll answer these...
4) Very different from RCCL/CCL. You have to understand it's an international mix of passengers and not everyone speaks English well, or at all. So the entertainment tends to be more along the lines of physical (e.g., acrobats/dancing) or singing. No real comedians (except sometimes slap-sticky), or "games" like Newlywed, Quest (or whatever it's called now), etc.
Also, the shows tend to be short...30-40min long.
5) The biggest difference is the passenger mix. Because of that, you'll often hear announcements in several languages. But, more importantly, it's a good way to meet people from different countries and learn a little about their way of life. If you enjoy traveling (outside of the country), you'll love this aspect.
Along with that, many report that the staff is more "European" instead of "American". Basically, they don't approach you 50 times an hour with a smile asking if you want/need anything. Instead, they "stand by" waiting for you to speak up and ask if you need something.
6) You didn't ask, but the Yacht Club is a completely different experience that really isn't matched on any other mainstream cruise line.
My personal opinion of MSC is that, once onboard the ship, it's amazing. In fact, until I sailed an E class with Celebrity, MSC ships were the most beautiful I'd ever seen. However, getting to that point can sometimes be incredibly frustrating. Pray you never have to contact customer service, or have anything "automatically and silently" changed by their computers.