Now it's faster to go to previously visited web pages because you don't have to type the full address: Opera Mini's auto-complete shows a list of suggestions when you type the first characters, like most desktop browsers. The new address bar can also be accessed while visiting a web page and it can be used to search the web. Opera Mini added an option to search inside a page so you don't waste precious time trying to find a certain section in the current page.
If Gmail's mobile application doesn't let you add attachments, Opera Mini 4.1 is there to help. You can now upload photos, videos and documents from your mobile phone to Gmail, Flickr or any other site. When you clicked on a link to a file that couldn't be handled by Opera Mini, the browser wasn't able to download it to your phone. Now you can download files and save web pages to view them offline. To use this features, your Java installation needs to include JSR-75, "an optional package which allows Opera Mini to access the file system on your phone".
Opera also tests some signed versions of the browser that let you change the permission settings so you are not constantly harassed by dialogs that ask your permission before reading from the memory or connecting to the Internet. Opera Mini's blog has more details about the signed versions.
The software is still in beta, but it should work on any mobile phone that supports Java applications. Opera Mini only has 152 KB because most of the processing is done on Opera's servers, which convert web pages into a simplified format that can be displayed by the application.