Coming to this much later in the development of the game, I'd say if you power level your first character, you'll miss out on a ton of content and story-line and the many parallel quests that go on. The writing is well done, and there's even a ton of additional lore to enjoy (although a separate lore reader app for mobile might be nice... it really is a lot of text).
Even without power leveling, I've breezed by some content because of leveling beyond it very quickly.
For secondary characters -- since each free account can have up to 3 -- the story is different. However, knowing that the end game content takes a lot of time with daily and weekly quests, and a bit of grinding, it all depends on how much you enjoy "the grind" once you cap each character.