I still can't comprehend how people seem to be able to relate a Six Flags park to a Universal park, they are practically nothing alike in any way.
Someone please explain what they have in common besides that they have a couple of outdoor rollercoasters
This one baffles me too. But I think that when people describe Universal as being "like Six Flags" in any way, it's generally driven by one of these five possibilities (or some combination of the five):
1. The person saying Universal is "like Six Flags" has never actually been to Universal. They may be basing this assessment solely on having seen a picture of the Hulk or Dueling Dragons, and they assume the entire park (or BOTH parks perhaps) to be filled with nothing but thrill rides.
2. The person making this statement may have visited Universal at one time, but did so with a very closed mind and determined not to like anything (thereby not giving it a fair chance, but entering the park simply so they could later make the argument that they did give it a chance). Sounds crazy, but I do think some fanatics think this way.
3. Since Six Flags is generally the benchmark for anything that is the antithesis of Disney, many people are in the habit of referring to any non-Disney park as being "like Six Flags". I think some people do this even in cases where a park has MUCH more in common with Disney than it does with Six Flags (which is definitely the case with both Universal parks).
4. Many people tend to simply repeat the opinions of others because it's more comfortable to be part of the group, rather than the independent thinker. Consequently, I think a lot of people just repeat the "Universal is like Six Flags" statement without even giving much thought to what they're actually saying.
5. I honestly think some hardcore Disney fans are threatened by the existence of Universal (or better yet, they consider Disney to be threatened by Universal), because Universal truly has the only parks in Orlando that come anywhere near rivaling (and in some cases they surpass) the Disney parks. I think these fans would rather have people believe that Universal isn't worth a visit, so they try to convince people that it's "just another Six Flags" or "just another thrill park" or the ever popular, "I saw both parks in 4 hours".

Like #2, I know this one sounds a little crazy, but I do believe some people think that way.