Interesting I just released a Manufacturers Generation in HTML Catalog for 4.0.x; Glad to see this, along with the detailed images dhtml popup are now going to become standard.
One thing I'd like to see in the search is a stop words list used, and perhaps and indexing system with fulltext features and not just %keyword% AND %keyword% as its obvious this similar setup doesn't even work here on phpBB's forums, it makes searching cumbersome, and thats one word I'd want to avoid with potential customers, most use the search functionality of any site to find what they want quickly. Here's some of my suggestions:
http://cart-lab.blogspot.com/2005/10/improving-x-carts-search.html
XHTML 1.0 Transitional compliant code, defiantly a plus!
Kudos X-Cart team, it seems 4.1.0 will take a turn for the better!