It isn't a huge dramatic change to me. v.50's for Opera have been fairly significant in the past, but this is an incremental update. I have been using all the developer builds and this seems as solid a release as O has put out in several years.
What is new? A thin changelog on the UI front (which is good to me. the less they mess with the drivers' controls - the better).
Password synchronization is nice. A long sought after feature that many of us have already addressed. I would rather enjoy the ability to just run a clean copy of opera out of a dropbox folder. There are ways to do it and I run my opera out of dropbox, but a clean way to stop cross-sync issues would be nice. Afterall, it isn't just passwords, bookmarks and cookies we want to sync - it is the actual browser code too. I have a main laptop, a netbook, home pc, and three work pc's. Keeping Opera up-to-date with the developer builds is a chore that dropbox fixes easily; however, you have to remember to close opera before moving on to another machine so that dropbox can sync them.
The new rendering engine is top notch.
The new speed dial stuff? It's not my deal. I don't use speed dial much. I find a list of sites much faster than having to look through a dozen buttons - much slower. The new "extensions" are baffling to me. I have read and read about them and I can't see that they serve any utility... I have missed the point entirely here.
Opera is now the only browser that passes my benchmark, Firefox 3.6 did though Firefox 4 borked that. While there wasn't a lot of new rendering engine related stuff it was enough to write a decent blog entry about unlike Firefox 5 which didn't justify a whole new version number. Opera's theme looks bad on XP (cue Microsoft employees denouncing XP) so I switched to the Windows native theme. They've fixed a few bugs, if they fix a few more GUI bugs it'll be a solid browser overall. In example the bookmarks toolbar folder isn't correctly synced with the bookmarks manager. Still a solid release.