The only reason why i sight wont work in opera is beacuse it has been designed badlt(ie with loads of IE hacks rather then real w3 code) or beacuse someone(usualy in the case of secure websites) hasn't added it to the list of acceptable browsers.
You used to be able to tell servers that you were actually using the latest version of IE which would mean that you could use opera on any site but i haven't found the option on the new version(or found a need to search properly).
AS for the argument that opera has its market share for a reason. Bollocks. Thats like saying aberdeen angus beef is inferior to big macs. People don't know better!
dave