Have to agree with SzGeri -- the selection method feels unintuitive, and the lack of major manufacturers cuts down the site's utility drastically.
Besides that, I think there are a lot more search criteria that could be added, the highest-priority of which would be kitting. Things like 40%, 60%, TKL, and Full-Size (along with 7U, Tsangan, etc.) are a big priority for many.
Some categorization of legends would also be great -- whether or not there are sublegends, what they are, and what kinds of modifiers (text, icon, or both) would all be super helpful. I know those would be hard to implement when trying to automate things, but it really would help go the extra mile to be a really great search engine.