• don't see the need and/or benefits of a good programming IDE like Zend Studio or Eclipse PDT
• have never used some form of version control like Subclipse
• don't use a consistent methodology
• don't use test-driven development
• don't return content but echo or print it from your functions or classes
• return HTML, not data, strings, or objects.
• don't allow intelligent error handling
• you think reusable software equals/requires your code to be OOP

Now granted, some of them are a bit more high level than others, but if you're not headed towards a lot of these, you might change paths, hop out of that comfort zone and branch out into the community and the language a little bit more.