So there are many different features that might be important to any given user. Personally, I think there are two very important considerations. The first is the quality of the flicker filtering. As you may know, broadcast video is an interlaced signal. When making a conversion from a non-interlaced computer signal to an interlaced signal, flicker is introduced.
Most people are highly sensitive to flicker and without proper filtering, viewing can be painful.
Manufacturers of higher end scan converters offer multiple filters to accommodate different preferences, since there is a cost for filtering in terms of lower resolution. Personally, I have a very low tolerance for flicker and I like to adjust for maximum filtering. But I have an associate who prefers a slightly crisper image and who is willing to tolerate a higher level of flicker. This is the reason for offering multiple filters. Specsmanship aside, three filters are needed to cover the useful range.