When I justify why I prefer subs almost exclusively over dubs, I usually argue that in the case of at least decent shows (I like to think I watch shows that are at least decent), the Japanese VAs exhibit more authentic or expressionistic emotion and character. Of the dubs I've seen, quite a few sounded like people who were still greenhorn VAs were reading off of a piece of paper are trying to "say this line" rather than "be this character".
Let me give you an example. Here's a bit from FLCL:
https://www.youtube.com/watch?v=CgllycqBk-w
Personally, I love FLCL to death and I'd consider it a favorite of mine and also my first explicitly Japanese anime I've seen (by that, I mean I'm not counting watching Pokemon or Yugioh when I was a youngster). This isn't a fantastic example since the dub isn't that bad, but you can definitely catch what I'm talking about. Some of the lines and deliveries feel like they're just "spoken" and not "felt" by the VA, as if they're just reading off of a paper they glanced at once or twice prior.
I don't know how much more dedicated Japanese VAs put into their work than the English VAs, but the Japanese VAs offer a more intimate expression of their character in most occasions in my honest opinion.