Besides the other past links that people have mentioned, you might also want to take a look at TheDamian's recommendations on setting up vim in Desparately seeking a bilingual vim/Emacs expert, and the resulting recommendations for emacs and BBEdit.

As for the advantages of a few various text editors -- vim/vi/ex or something similar are installed on just about any unix-esque system. ex has advantages when your terminal type is screwed up, or you're coming in through a serial console, but I don't recommend line editors for serious coding efforts. (but they sure come in handy when you're trying to fix something that's broken).

If you're on a mac, I highly recommend BBEdit or TextWrangler, if you can't justify the cost. (Unfortunately, the current versions of them require MacOS 10.3, and they don't have older versions available from their website, for those of us still using MacOS 10.2 or earlier.)

As to the features -- well, it's really hard to say, as once you get used to using a program, you start taking them for granted, and forget about what it was like without them. I know that I make heavy use of its find/replace (using PCRE (Perl Compatible Regular Expressions)). You can also write scripts or filters to work on your files (again, using perl, if you wish), and you can add extra functionality with AppleScript. There's also syntax highlighting, split screen editing, notification of mismatched parens/braces/brackets, a menu to quickly jump between functions, integration with a few source control systems, and a whole bunch of other things that I'm forgetting about.

Disclaimer: I have been a beta tester for BBEdit, so I may be biased in my opinions