Few reasons:
– problem with displaying images, in some cases images change positions on the website during uploading the content. Finally images go to the proper place but there is some delay in interpretation of css
– some input forms with option “upload file” doesn’t work with Opera, I don’t want to go into technical details, but it is serious technical problem :)
– what I found today: just wanted to log in to Google AdWords (software to advertise in Internet, known as Google Pay Per Click): We’re sorry, the AdWords interface does not work with Opera. We recommend using Firefox 3, Internet Explorer 8, Safari, or Chrome to manage your AdWords campaigns. Learn more.