Sometimes I wonder if the whole angelic theme puts off people from the series. The only aspect of the theme that seems to generally get positive reactions is the "falen angel" idea with Brazila. Every other attempt Goseiger made at following it seems to be met with mixed reactions. Initially, I was annoyed by how mixed the execution of the angel theme felt, with the cards and animals thrown in, but, now, I wonder if a pure angel theme would be just too hard to do...
I know you're speaking of "in general," but I really loved how they did the angel theme. It's been said before, but I'm pretty sure the idea isn't that they're, like, "Christian Angels." They're Guardian Angels; effectively closer to the folkloric concept of angel than the dogmatic religious one. (I really wonder if people would have gotten the series more if TVN had just translated "Gosei" as "Guardian." I'm pretty sure that would have worked in all cases that the word is used.)
Once you move past the idea that there "should be" some aggressive religious imagery going on there, I think the cards/animals stuff is easier to accept. (It's still not
awesome, IMO -- I'm not fond of cards or animal mecha in the best of times -- but it feels like less of a total record-scratch. It's not as if guardian angels really have a lot of obvious imagery to associate with them, so it's easier to fill in the blanks with whatever you need. Take this in contrast to Pirates or Wizards or Samurai.)
Once you think of them as Guardian Angels, more about the series falls into place. The characters aren't "deep" because they aren't supposed to be. They have their problems, but their internal struggle is framed entirely around "how can I get better at protecting Earth and its people," which is certainly aspirational, but not a struggle most normal people face.
So the power fantasy isn't "wouldn't it be cool to be an angel like these guys;" it's "wouldn't it be cool if there were guys like this out there looking out for us," and we watch as they learn and fight and become more and more the type of people you'd really want watching humanity's back. The concept is a bit more like Ultraman does (from what I've seen of it) than what Kamen Rider does. That's why Nozomu is there, to be the
real identification point and make us think how nice it would be to have awesome special friends looking out for us in bad times.
It's an off-beat thing to do given the fan preference for main characters who are "real people with real problems," and obviously the series didn't always succeed at portraying what I described (shuffling Nozomu off-camera constantly during the early episodes didn't help), but in its platonic ideal form I find Goseiger's concept pretty exulting.