It's obtrusive. Try getting a program running while watching a DVD or a YouTube video or worse yet be playing a game in full screen mode and accidentally nudge the windows key. It's like driving down the road and sudenly the airbag spontaneously goes off in your face. Many people arguing against the Metro UI are siding on the side of being inconvenienced with having a menu system take up the whole screen instead of the minimalistic and convenient menu bar that is confined to the bottom left of your screen (or where ever you set it to be). I'm sure if Microsoft kept with the start button feature. along with updating the theme to have the Metro feel (Using the font style and color scheme), there wouldn't be an uproar over the MetroUI.
The bottom line is that we're not arguing that Windows 8 is the worst performer in terms what the OS does (faster, efficient, etc.). We're arguing on the platform of convenience. I understand that there are shell replacers. I'm sure that many of us or knows someone who wouldn't be that savy to know how to install them or the existance of them and/or disable MetroUI, the charms sites.
That's perhaps the jist of the arguments going on while lurking.