My Opera Forums » Opera Topics » Open the Web » Google Mail!
As someone mentioned already, the problem is that Opera does not support XMLHttpRequest to let a JavaScript request files from a server. There are other ways to implement similar functionality (for instance add SCRIPT elements to a document through the DOM or load content in hidden frames) but it seems XMLHttpRequest is being quite useful.
XMLHttpRequest is a functionality not covered by any standards. It is invented by Microsoft and also implemented in Mozilla and other Gecko-based browsers (though slightly differently since MS's implementation uses ActiveX).
Because it is non-standard and has not been widely used before, it has not been considered very important to implement. Since a major site like GMail now starts making very good use of it of course it will have to be supported. I can not promise when but it will happen. So, rest your arguments - we will be working on it :-)