Some of the features that were not mentioned above:

• syntax highlighting. I like the visual cue when I mistype a keyword, forget to close a literal string with a terminating quote, etc.
• code completion. That is have a mechanism to complete the variable name I am typing, as long as it has been declared before.
  This could be very useful when calling methods on objects, etc.
• while calling a subroutine, displaying the prototype if one has been defined.
• some kind of template feature, so that if I am beginning a new script, then it would have the standard use statements I have in all my script. (eg: use strict;use English;)
  Also, useful for adding new modules, oo-packages, etc
• regex search and regex search and replace.
• unlimited undo
• able to interact with some of the common source code control systems (eg: CVS, RCS, perforce)
• Able to do compare files {BTW, EDIFF rocks :-) }
• It should be customizable and extensible, preferrably using perl as the script language.
• cross-platform support. I would like to use the same editor whether I am working on Linux, FRE-BSD, Solaris, Tru64Unix, Windows, VMS or any other OS that comes along.

The popular editors (x)emacs and vim (I don't think much of the plain vanilla vi) do have most of the above things but not all and not as well as I would hope.

Just my .02