The first thing that you'll recognize is a somewhat large banner containing advertisements. This banner can be controlled to show specific advertisements. But I do not recommend that because you'll lose some of your limited anonymity on the net. The navigation buttons are familiar and most of them work the same way as in Netscape. However you'll notice the lack of a STOP button to cancel a transfer.
Opera uses some sort of multiple document interface.
Contrary to Netscape, all document windows will be opened in the mainframe as subframes. You'll notice that you have seperate buttons for each open document. This allows you to operate Opera even if theres no windowmanager running. A very nice feature, borrowed from the MS IE, is the ability to switch to a real fullscreen mode. Press F11 and the current browser frame will be shown fullscreen, really filling the full screen, not only the subframe of the Opera window.