Just remember, Google doesn't like too long alternative text in images. Take advantage of the following two Firefox Browser add-ons: Senseo (you'll need Firebug first) and KGen. Then check the resulting keyword-density for important keywords. Maybe you'll need to provide a longer product description.

The canonical tag on errorpages make no sense! For gone webpages just setup a 301 redirect in the .htaccess. And make use of the Google webmaster tools to identify waht Google sees.

Take your time to create realy valuabe titles. Don't mess with titles. And i agree with ninjamarketer: You will need to make enough changes in the content to prove the need of title changes. The meta description has little effect in SEO. But great effect on users. It's ok to change the description in order to achive a higher conversion.