I've been a professional web developer for 18 years, and I've tried just about every editor and IDE out there for the Mac, and trust me when I say that for sheer power and functionality, nothing else even comes close.
Syntax highlighting and autocompletion for *everything*: tags, attributes, classes, IDs, filenames/paths, just about every thing you type, and in just about every dev language known to mankind. Optional file watchers for compiling preprocessor languages like HAML, Sass/Less, CoffeeScript, etc. into final HTML/CSS/JavaScript code. Powerful code refactoring: change a filename, function name, variable, class or ID throughout your project. Select just about any identifier, and it highlights every other instance; it also highlights mistyped or invalid identifiers, validates in near real time, graphically shows the scope of every nested element, matches opening/closing tags and braces… if you pay attention to all the hints it gives you, it's nearly impossible to write wrong code.
There is still some room for improvement: the project file browser is pretty rudimentary (doesn't show timestamps or file sizes, D'OH!), and the built-in FTP is sometimes a little flakey, but in whole, it's a pretty amazing tool for the web development professional.