Specifically, mobile-friendly typically disables zoom, so that zooming can't be used to choose text size or to scroll through the page at a higher rate. Instead, everything is reduced to a kind of very long ticker-tape, where if you're deep into the page, you need to scroll many, many times to get anywhere.
Alternatively, you get a page broken up into expandable sections, like mobile wikipedia, where it's very tedious to visually scan the page. Wikipedia is one of the best examples of a site where I prefer the desktop page on mobile.
Another issue is limited functionality. Sites with complex forms typically present a hobbled or otherwise limited, basic mobile experience.