Checkboxes vs. Multi-Select
I'm quite a fan of the Chosen jQuery plugin (http://harvesthq.github.com/chosen/) for multi-selects. It also handles the related problem of single selects where there are an enormous number of options to scroll through, by adding a textbox for a filter search.
While we can certainly review some core forms for having "too many" multi-select options, it's not sensible to assert that a multi-select with 'many' options must, by definition, be badly designed. In many cases the number of options that will appear in the final form is entirely down to the user or the wiki administrator. So we might want to review the watchlist-edit form because users commonly have 'many' pages there. But there is no reason why a wiki might not be perfectly legitimately configured with hundreds of namespaces, and suddenly all sorts of filter selections have "many" options.
