Thanks to James and Robert for this addition, though I doubt I'll ever have a chance to experience it.
Obviously, the top 20 parks on attendance make it. So do the top 5-6 international parks which significant English-speaking followings.
That leaves five spots for parks that I think are significant enough to merit discussion and that will draw traffic to the site by being listed.
At one point, we tried to get as many parks as we could on the website, and traffic tanked. The presence of all those smaller parks diluted the prominence of the big parks, leaving their fans to click elsewhere. So I culled the database and site traffic rebounded.
Since then, I've gradually added parks back to the mix. Worlds of Fun getting a new coaster this summer puts them on the list for 2009.