D. Gray-Man > Blue Exorcist (sorry, man)
Why must you hurt my feelings like that?
Kidding. I actually have been wanting to try out D.Gray-Man as it does seem it's something I will like. *Insert "why not both?" gif here*
In regards to Blue Exorcist, the anime does kind of suck since they pulled an anime original ending. The manga is much better and is still ongoing.
Anyway, there are plenty of anime and manga titles that clearly are influenced by older works, employing similar concepts in their stories. I don't think it's necessarily bad, but I think the mentality that anything recent is better than older stuff is simply wrong, as works should be judged on their own merits.
This goes for any kind of medium, not just anime or manga. After all, human creativity and imagination is limited to existing concepts and combinations of those... There's a reason why works are categorized into genres.
For people who have seen older anime and manga, there may be a lot of modern works that feel like rehashes of older stuff. Newer audiences won't find them like that, though they still should be aware that, more often than not, whatever they're reading/watching is something that may have been done before and acknowledge that.
In the same way I think newer fans should explore older works, older fans should also try out the newer works. Otherwise, they're in no position to criticize anything.
Drawing a parallel with video games, there are people who got introduced to platforming games with Super Mario Bros., others with Sonic, others with Rayman, others with DKC, others with Super Mario 64, Galaxy and so on. Yet, I don't feel that any of those games undermine the others.
Well, I did end up writing more than I imagined... I think I will be off for now given it's late night where I am.