I've always heard that things go in cycles. What was once out of fashion 20 to 30 years ago goes back into style. I guess that explains the popularity of Gatekeepers.
People always talk about how cliche anime has become and how unoriginal things are.
There are those titles that just copy successful series and don't do a very good job of it like Bubblegum Crisis 2040. Then there are those that just make fun of the trends, which gives them an excuse to be completely unoriginal like the Magical Girl Pretty Sammy and to the more extreme level, Excel Saga. But then there are those that take old ideas and do something very different with them. This can be seen in Neon Genesis Evangelion, and my current favorite series, Gate Keepers. But no normal human can stand up to the Invaders, so this secret force must find humans with special powers known as a Gating Ability.