I don't expect all features of every text editor in PL/SQL Developer. PL/SQL Developer is very good. The most common things I use Notepad++ for are:
xml viewing for CLOBS. I used to use a different editor, but my workplace is starting to limit what we can use.
I know PL/SQL Developer has column editing, but I like how Notepad++ does it better.
Double click a table alias in SQL and they all highlight, which helps me find things in large SQL statements.
The "find all" in Notepad++ opens a window at the bottom that lists all of the line numbers in the document with that text (It looks and works kind of like the compile errors window). Sometimes that is nice.
The command window. The Command window doesn't have line numbers, I cannot see bookmarks and stuff like that. Sometimes, I just change it back and forth to/from a SQL window for that though.