Man, I guess I will get in here before the lock, but it is only natural for people to make comparisons between new things and things they are comfortable with... Fear of the unknown...
All of us here agree that IIDX is pretty different to guitar hero, but like someone else said, it is fair enough to compare two games which both require you to press buttons in time to music.
For example, I am a metalhead and can clearly see that there are many genres of metal within metal. Try explaining that to my parents though and they would put metal together in one single group, even though I know that early 70's "metal" like Deep Purple is considered pretty tame and a long way away to modern extreme death metal like Dimmu Borgir for example!
People like us know the ins and outs of the music genre as we are fans of the ultimate music game, albeit one of the most niche music games. This means that we have to overlook the opinions of people who think that IIDX is a Guitar Hero rip-off or is shit in comparison as they simply do not know any better.
The slightly politically incorrect statment that some opinions hold more weight than others is probably pretty valid here.
By the way, this argument is kind of old anyway, it began right when Beatmania US was released and some review threw it away as a Guitar Hero clone.
Lastly, it doesn't matter how hard you try to get other people into something like IIDX, it rarely works. Most people are not in to this sort of gaming unfortunately. If you look at Japanese society, they generally still enjoy the arcade style games where people compete for high scores because that is how they look at gaming (among other things).
If you look at western games and western attitues towards gaming there is a high emphasis on game completion by getting to the end of an adventure or something, not repeating something over and over again to improve your score or master something.
Guitar Hero sucks, but if people want to play it, let them go ahead, and if they are not interested in IIDX then there is no need to get angry about it. Some of us just know better!
