Yeah, the original one is-- Think of the characters' clothes (somewhat), expressions and activities such as dancing generic 1960's dances to generic 1960's music. Some of the more recent incarnations aren't dated yet, but they probably will be eventually. Scooby-Doo's not exactly renowned for its clever, timeless writing.
I'm not trying to say the original is bad or anything, I enjoy watching it a bit, but it is fairly dated.
Speaking of dated, I just made a post in the "Under-rated Cartoons" thread about Rock and Rule, and it applies here too. That's a very dated movie, largely for the same reason something like the Jetsons or Back to the Future II is dated-- if you try to set something in the future, parts of your current culture will unavoidably seep into it. Hence the Jetsons everybody listens to 60's-style jazz and says things like "galactic groovy", and in Rock and Rule all the rock music played sounds like it's from the late 70's. It's still a fun movie though.