This is from "the potentially don't know what you're talking about" department, since we have NOT sailed on MSC (we shall do so in June 2009). However, we do travel a lot, and we do participate in many travel websites. My two cents worth: expect bad reviews in about a 5:1 ratio (to good ones). Sadly, our observations indicate that there are a great many people who complain, at great length, over what most of us would consider to be minor details. I'm sure you all know people like this. Sigh. So, we read the reviews with a great many grains of salt, look closely at what they specifically complain about, and we focus more on the good reviews than the bad. AND, we understand that the quality and enjoyability of our vacations depends mostly UPON US, upon our attitudes, and how well we adapt and roll with the punches.
Travel is an adventure, and no matter what may befall you, it is all part of the adventure. You can relax and enjoy it, or you can tighten up and whine. The choice is yours.