1. The default SilverStripe search form only searches page titles and content field (as in SiteTree). How can you limit the search to only a custom field added to all page types? This would enable the site admin to define exactly what can be searched on any page.
2. In the default SilverStripe search form you cannot search for common words and anything of length 3 characters or less. How can you remove this restriction? Following from point 1 above, the site admin may wish to define 3 character abbreviations as searchable keywords.
The search character limit is a MySQL issue, as for searching extra fields things like the forum have got there own custom search forms with their own querying engines.
Search is one of the areas that is going to get some love in the future, in the mean time have alook at how the forum does its own custom one and sort of play around with that and see if you get anywhere