1. Panel customisation. This is my best feature. I can put any button I want almost anywhere, from a clock to the enable cookies preference setting, it can all be turned into a button and placed anywhere.
2. Tabs/Panels. The tabs can be turned into windows and (for example) put side by side, the number of times I have had to open two firefox sessions to do just that...
3. Undo. You can undo almost anything you have done, including closing a tab.
4. Session Management. My session management in firefox kept crashing, it works wonderfly in Opera. Now I can save all my tags.
5. Fonts and Rendering. The fonts and general rendering quality is excellent.
6. Images. I can do all sorts of tricks with images, for example only cached images used as links can be displayed, or text browser mode can be enabled. This really speeds up some of my regularly used slow sites.
7. Certificates. When I connect to an https site it shows the organisation name and location of the certificate in the URL bar.
8. Integrated RSS feed reader and mail client and IRC client. I have only done brief testing of these but they are quite usable.

There are some features I miss and some downsides however:

1. Livebookmarks. These are very useful, particularly for my del.icio.us feed. I miss them.
2. Cookies. When you disable cookies in Opera it also disables stored cookies. I want to be able to make a whitelist like I have in firefox. I am sure there is a way to do this.
3. Functionality. My blog's WYSIWYG editor stopped working in Opera.
4. Non-Free. Opera is free as in beer, what's more the free version will deliver an ad stream to you. I chose not to get Google website specific ads and went for the default stream which gets blocked by our organisational proxy so all I get is an irritating grey block in my top right corner.