Update, 2nd August 2008
Suddenly it seems that the KML export from My Maps no longer pages like it used to, so this process is now a lot quicker!
Just use the link ‘View in Google Earth’, and paste it directly in the search box to get access to the geographical culling method. No more going and creating a kml file or any sillyness.
In order to keep the map working at reasonable speed Google Maps needs to limit the available features visible at once, quite simply the more features shown the slower the map gets.
For MyMaps the features are split into pages, so that a reasonable amount of data is shown on one page – effectivly enough that will work at a reasonable speed. This normally results in between 60-200 features at once, but sometimes that is not enough.
However Google Maps uses a different limited method when displaying KML files, a regional geographical culling based method. A certain number of features are shown, to give a reasonable overview, but then as you zoom in more features become visible. This often results in more results being visible, but importantly doesn’t impose the arbitrary pages, so its often more friendly on the user, as can just zoom in on an area rather than flick though pages to find their area.
I’ve moved the meat of this post to a thread on the Google Maps Group – let Google host it!