America's theme parks are dominated by the companies Cedar Fair and Six Flags. Their theme parks usually focus on the rides rather than the theming, hence coasters in America are (mostly) considered better than in Europe. (There are, I believe, also more in the USA) Some parks (Busch Gardens Tampa & Williamsburg, Universal Studios and of course Disney parks) do offer great theming though, but their rides are considered less good than the ones at a park of Cedar Fair/Six Flags.
In Europe though, most good rides are greatly themed. E.g.: Taron, Black Mamba, Troy, Baron 1898, Dragon Kahn, Shambala..
But also a lot of them aren't: Helix, ISpeed, Silver Star, Goliath, Expediton GForce (just to name a few).
However these rides are located in parks with the focus on theming as well (OK, maybe except Goliath). Theming is IMO a big difference between USA and Europe parks. USA rides (without any theming) are great, but if you combine a great ride with theming you get something awesome ! yeah I'm looking at you Taron, you masterpiece..
People who aren't into coasters probably didn't get all these names [blah] sorry for that, on this kind of topic I can't help but talk about coasters [wink], but I hope my point is clear.