Looks like I’ve been accepted to speak this year at WordCamp Philly! I’ll be rambling on about the “Seven Deadly Sins of Theming” — a look at the ghastliest and most horrifying grievances that are made against best practices when making themes — specifically those for commercial consumption.
My main bullet points (so far) are:
- Copying Snippets (without understanding what they do)
- Bundling Plugins (just slapping them in your theme directory)
- Assuming Plugins (calling plugin functions directly from your theme without checking that they are available first)
- timthumb.php (using it when the existing WordPress thumbnails functionality is secretly better)
- CDN-Hosted JS, and using it the wrong way.
- Hardcoding CSS & JS links (rather than enqueueing them properly)
- Ignoring Child Themes (or, why your template directory is not the same as your stylesheet directory, and the importance of knowing the difference)
- Failing to take internationalization into consideration
- Hacking Core Files. (I’ll punch you in the throat through the internet if you do it)
I’ll probably be cutting or merging a few (to keep it at seven) — but please feel free to make any suggestions of egregious violations that you’ve seen in the comments section below!
George Stephanis 
Solid list!