What makes TextMate a good editor?
Download a free 30-day trial and find out.  The site goes into detail on all of the features listed below but these are the main features.  


Ability to Search and Replace in a Project
Auto-Indent for Common Actions Like Pasting Text
Auto-Pairing of Brackets and Other Characters
Clipboard History
Column Selections and Column Typing
Completion of Words from Current Document
CSS-like Selectors to Pinpoint the Scope of Actions and Settings
Declarative Language Grammars for Graceful Mixing and Hacking
Dynamic Outline for Working With Multiple Files
Expand Trigger Words to Code Blocks With Tab-able Placeholders
File Tabs when Working With Projects
Foldable Code Blocks
Function Pop-up for Quick Overview and Navigation
Plug-able Through Your Favorite Scripting Language
Recordable Macros With No Programming Required
Regular Expression Search and Replace (grep)
Run Shell Commands from Within a Document
Support for Darcs, Perforce, SVK, and Subversion
Support for More Than 50 Languages
Switch Between Files in Projects With a Minimum of Key Strokes
Themable Syntax Highlight Colors
Visual Bookmarks to Jump Between Places in a File
Works As External Editor for (s)ftp Programs
Works Together With Xcode and Can Build Xcode Projects