I wonder why Opera isn't more popular than it is. It's easily as good as any of the other big players, and packs a lot of features into a relatively small amount of space. It doesn't have to be #1, but I'd expect better than the ~4% it carries.
Opera's biggest problem is its low marketshare. Almost all browsers have little tweaks and hacks you need to throw in on occasion to get it to work within a certain browser and people just don't put these in for Opera. That's why you get site giving either bad code to Opera or not rendering properly. I can't tell you how many times I log onto a site and it says: You're using an outdated browser please upgrade to Firefox 4+ Chrome 15+ etc etc. At times it gets to be a pain in the ass, and for an average user who doesn't really love the browser, who wouldn't change?
On top of that they don't advertise their features. It has adblock, greasemoney, stylish, mouse gestures, no script all built in. Among many other features. If someone who is a diehard Firefox fan searches their extension lists and doesn't see that stuff are they going to take the time to find if the browser does have these features? Most people would not.