I'm disappointed in the lack of love for Opera in this thread, and it seems for many users that the primary cause is that they "haven't used it much." To me, this is extremely disappointing and very representative of the fact that in terms of market share/quality, Opera scores BY FAR the worst. If more people would give Opera a serious test run, I'm very convinced that it would have a way bigger userbase.
As m0nkfish pointed out, Opera is always the innovator when it comes to new features. Several people have made comments in this thread about certain browsers having certain functions, and I'm sitting here going, "yeah, Opera has that." Opera has virtually every worthwhile feature that any other browsers have. The same isn't true in reverse. Opera Turbo is an awesome tool and allows me to view Smogon without much trouble during its notorious downtimes as of late. Dragonfly is a great debugger, and while I'm not much of a programmer, I can easily recognize the usefulness of this for anybody who is.
Spatial navigation is an Opera feature that makes it way easier for laptop users to navigate the browser without a mouse that I'm certain many Firefox/Chrome users don't even know exists. If your keyboard happens to be broken as well, you can navigate Opera with
voice control. Opera users are almost always the first to have access to these kinds of things. After that, a similar Firefox extension usually comes out shortly afterward, with Chrome and Safari eventually getting the feature in a future release. In terms of features, Opera > Firefox > Chrome > Safari.
Default features. This is something that really isn't stressed enough. I hated that after I downloaded Firefox, it did almost nothing that IE didn't do. Opera has only a very basic "widget" system, and widgets are almost totally detached from the browser itself. Opera doesn't depend at all on third parties to develop its features, which is actually very important. One of the biggest flaws of Firefox is that, as you add features, the browser becomes noticeably slower and sometimes two extensions will conflict. Opera completely avoids this problem by having all of its features by default. And when I say default features, I'm not just talking about an adblocker, a spellchecker, and mouse gestures, I'm also talking about an e-mail client and and IRC client (which I use exclusively). Default features: Opera > Chrome > Safari > Firefox.
Customization. In Opera, damn near
everything is customizable. This is a problem I had when using Safari and Chrome, mostly, but Firefox too. (Actually, Firefox is almost as customizable as Opera, but my big gripe is that retarded status bar. I can do whatever I want with the rest of the toolbars, but the bloody status bar is inexplicably non-customizable! I ended up disabling the status bar completely in Firefox, because it's an ugly grey bar that got in my way, but that left me unable to see the status field. With Opera, I can take the status field and put it wherever the hell I want before disabling the status bar.) I don't really have a lot to say on this point, actually. I can move stuff around wherever I want, and if that doesn't satify me, I can edit anything manually via opera:config. I've used this to get exactly what I want in a web browser: a ton of features that don't get in my way. I have a short screen and I open run a ton of tabs simultaneously (something I wouldn't risk in Firefox), so I hate having tons of toolbars at the top or bottom of the screen and I need space for those. I keep my browser as slim as possible, and when I say "slim"
this is I'm talking about. I keep everything that I need in a sidebar, which only appears when I want it to, and the rest is compacted into one menu in the top-right corner. Opera > Firefox > Chrome > Safari in this department.
Opera has fabulous security features. I'm not terribly well-versed in this area, but I've still done my research. Any security flaws in Opera are fixed about as quickly as they are in Firefox (impressive, considering Firefox is open-sourced.) Safari has been known to have its issues, and Chrome has been called out on one particular issue (relating to Javascript if memory serves; I'm not sure if they've fixed that, but it was quite a long time ago so I would assume so.) Firefox has the edge here in my opinion, as Noscript is virtually flawless as far as security goes. I'm actually kind of surprised that Opera doesn't have a Noscript-like feature, but if they haven't done it by now they likely aren't going to. Firefox (with Noscript) > Opera > Chrome > Safari.
Standards support. Obviously this is no contest; Opera's always had this one. The only question here is the one of "why do websites like Facebook fail in Opera," and the answer is because Facebook was poorly designed when Opera had that problem. Opera now has no problem displaying almost all modern websites. Opera > Chrome > Firefox > Safari.
Speed, raw speed. Based on my personal experience alone, I'm willing to acknowledge that Chrome is very fast as well, but most test results show that Opera is consistently the fastest browser. That's of course before you count Opera Turbo, which can double, triple, or quadruple your rendering speed at the cost of image quality. Boo hoo. Opera is fast at everything. When I rate this one, I won't count "tests," especially as I'm not planning to cite any, but instead just go off of personal experience: Chrome = Opera > Safari > Firefox.
I honestly don't understand how Opera Software managed to produce the biggest browser in terms of features while still keeping it lightweight. Opera really blows my mind, and I urge anybody who hasn't seriously used it to give it a try. Of course, there are those who won't use Opera on the moral grounds that it's not free as in freedom, and I totally respect that (I'm actually a proponent myself; Opera is the only non-free software I allow on my computer). Opera's ideals aren't totally off the mark, though. Several smaller aspects of Opera are open-sourced (such as Dragonfly) and I tend to believe that Opera Software would love to make their product free, and that it's just not economically feasible for them right now.
As a regular internet user and a curious person who has previously done extensive testing on all five of the major browsers, I think I'm fairly unbiased, or at least I was, back when I was just experimenting with non-IE browsers. Opera has proven to me consistently the best in almost every category I've considered (I'm sure I missed a bunch here, I may go over this post tomorrow). I know Opera isn't perfect, and I do use other browsers occasionally. I use Firefox regularly because I prefer the way it handles SCMS; I use Chrome when... actually no, I can't remember the last time I used Chrome, nor can I think of any reason to use it over Opera. Heh.
Things I like about other browsers:
Firefox: as noted above, I was unable to use the shortcut from on-site pages straight to SCMS in Opera (I'm sure it can be done, but I wasn't willing to invest the effort when I'm sure it was intended for Firefox). Has better extensions than Opera, obviously, but I've already pointed out that I don't really consider extensions a good thing, especially when they're required for most of the features that Opera has by default.
Chrome: One process per tab is pretty neat, sure. Really though, Opera almost never crashes, and its session restore really nullifies the usefulness of running one process per tab. Firefox has session restore too if i recall correctly, so I really don't see all the hype about this feature. Chrome is almost as easy on the eyes as Opera, which is also cool.
ps: Since Zy mentioned me earlier in this thread, it would be rude not to mention him. Hi!