(Note: These are observations of a person who has not been to a Six Flags park... just impressions from the out side looking in)
Mistake 1) Names of parks.
Okay, so Six Flags has "brand recognition". However, why do they have to go and re-name some of the parks they take over? Riverside Park becomes Six Flags New England, Gaeuga Lake becomes Six Flags Ohio (now Six Flags Worlds of Adventure), Adventure World becomes Six Flags America (okay, in that case it is an improvement.) I like Cedar Fair's method... buy the parks, but keep the original names (Cedar Point, Dorney Park, Valleyfair!).
Mistake 2) Names of Rides.
We've been here before... but its bad enough they have all of those clones, do they have to name them the same thing? How many "Superman: Ride of Steel" do we need? It would be one thing if they would put these in a park on the east coast and one on the west coast, but they have three in the north east (MD, MA, NY)
Mistake 3) Mass Produced feel
How many clones can you have.
Mistake 4) Replacing the history of the old parks with the new mass produced feel.
Before Six Flags took it over, I had a desire to go to Geauga Lake... now that desire is gone since it has been taken over and made over in the Six Flags image.
Of course, when you come down to it, is this a great problem? Not really. The GP sees the Six Flags name and they will flock there... just like they flock to McDonalds. They see that familiar name, say "Six Flags? They have some good parks... lets go," while at the same time letting the local, independent parks relying on the local population for their support.
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"Resistance is futile... you will be assimilated." The BORG's (and Six Flags') motto.