There is a "paste from Word" button. When you're using the WYSIWYG editor (Visual tab), there's a button that has a bunch of colored dots on it. When you hover over it, a little box tells you it's called "kitchen sink". Click it to expand, and you'll see a button with the Word icon. Use that to paste in Word content. It's not 100% effective, but it's better than direct copy/paste.
The problem with copying from Microsoft Word is that the characters used to create the content in a Word document are specific to the Word Program. Other programs won't understand many of the characters and "code" MS Word pops in there. (You can always tell a site that has someone who wrote the content in Word and pasted it onto their site - ever see those weird black diamonds with question marks in the middle? or the capital A with weird characters in odd spots? That's a "pasted from Word" content bit. the browser can't render the MS Word-specific characters. it's kind of like popping a English-speaking-only American in China and telling them they have to teach a group of people how to cook a 10-course meal in 20 minutes - in Chinese. The American would be like "Huh?")
I always tell my clients not to bother writing their content in Word. Write it in WordPress and save yourself the headache of trying to fix all the junk/bloat Word puts in there. The Visual editor already has the buttons you need to do what you need, and you don't get the bloat.