Opera is one browser I have always wanted to like and make my primary browser but have never been successful in doing it. The browser is fast and has great features but a few things have always prevented me from making it my primary browser -
1. No RSS live bookmarks. Once I got a taste of live RSS bookmarks in Firefox, it was hard for me to read RSS feeds in any other way, no Google Reader, no RSS reader would do it. I don't want to open a separate window/program and pile up RSS feeds in there. I love it the way it is in Firefox. You just read them in a drop down menu off the bookmarks bar and they automatically get discarded as new ones come in.
2. Website compatibility - This may not be Opera's fault but nevertheless it works against them. Lot of sites still don't work right in Opera and some flat out refuse to proceed unless some other browser is used.
3. Per site default zoom level - These days screens have high resolutions. Lot of sites show up as very small text (Tom's Hardware is one, another in NY Times). In Chrome when you zoom a website, it remembers it forever. The next time you go to the website, Chrome shows it at the zoom level you set earlier. It remembers different zooms for different web pages. Opera has just one global default zoom setting that applies to all websites and actually lot of websites look terrible when zoomed in Opera (try Tom's Hardware). Also, Opera's rendering of input search boxes get screwed up when a web page is zoomed. Try editing a query on Google when the web page is zoomed and you'll understand what I mean.
If they fix these three issues, I would make it my main browser immediately. For now it has to be Chrome (sigh!).