Here's the top of my CafePress enhancement wishlist:
Validating HTML keeps browsers out of quirks mode and makes cross-browser compatibility much easier to deal with. This is especially important when using CSS for layout. It also helps ensure that pages are spidered effectively by search engines and accessible to users with disabilities. Besides, it's just the right thing to do for the health of the web.
Currently there are elements on the page which are hard to modify with CSS because it's difficult or impossible to find a CSS selector that targets them exclusively. This forces unnecessary use of overly complicated JavaScript for DOM scripting.
It would be nice for performance reasons to get custom CSS out of every single page load. It wouldn't hurt your bandwidth bill either. I know it's possible to load stylesheets from an external site but I would prefer to not have my CafePress shop dependent upon another site.
Titles and meta tags are important for SEO purposes and should be unique for each page. CafePress actually does a fairly good job with page titles but having the same meta description and keywords on every page is killing me. It's especially problematic for meta descriptions. These appear on SERPs. I'd ALMOST rather not have a meta description at all than have my generic store description on every page.