I'm always impressed with the proactive way DL gate agents handle boarding in the U.S for domestic flights. They are constantly on the loudspeaker making detailed, clear announcements so everyone knows their place.
In Europe things are different, there are more languages to deal with and gate agents in AMS and CDG are definitely too timid. They seem to hide away until boarding begins and make one or two short (inaudible) announcements that people barely listen to. Add to that badly designed boarding gates, poor signage and it's no wonder pax accidently go into the wrong line, which obviously also means longer SP queues.
Perhaps staff need to call boarding depending on certain criteria. E.g. if more than 8 pax in C, call C first, otherwise don't bother with the hassle of shuffling people around.