Working with or against your Mac and Mac OS X With Opera and Firefox being ports, they have a tendency to favour proprietary technology.
Therefore, instead of using Keychain and your Mac's built-in spelling dictionary, they have their own equivalents.
Firefox is also far from a good Mac citizen, and although Opera's interface impresses and feels at home on the Mac, its cross-platform roots are betrayed somewhat when you start digging.
As a Mac-only browser, iCab behaves much as you'd expect, and its tab exposé is a nice touch, but the general interface is ugly, often making the browser feel almost as alien as Firefox.
Safari, being an Apple app, integrates flawlessly with other technology in Mac OS X; and although Camino shares a layout engine with Firefox, it mostly feels as native as Safari, utilising Address Book, Keychain and Bonjour.
OmniWeb is also generally a good Mac app, although its drawer-based tabs and tiny preferences content feel like a relic from a bygone Mac age.