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I don't think the season was particularly lighthearted - I mean, (black) humor has always been part of the X-Files. I think it was just a plain old bad decision, and the elapsed time between season 9 and 10 that's somewhat diluted the showrunners' sense of what made the X-Files the X-Files. Like with Arrested Development. I don't know what the general consensus is of season 4, but I personally thought that major elements that made the first three seasons great were lacking or plain absent in the fourth. For instance, the dynamism that resulted from the Bluths constant proximity to each other (whether that's in the form of them congregating at the model house or the parents' apartment or wherever) was almost entirely lost due to the decision of splitting them up to tell their own individual stories. If the show hadn't been cancelled, I think the hypothetical fourth season in that scenario would've turned out to be very different.
Anywho, I'm getting majorly off-topic.
On topic: still reading Kenichi. After I'm done with that, I'll be moving on to Noragami, which is bloody fantastic, by the way. I love how Toka's popularized and streamlined Shinto mythology in it. The cast of characters is intimidatingly large, but I think she's doing a great job of fleshing it out - Yato and Yukine are particularly great in that regard. I also really like her art and its subtle stylization; it's sheer eye-candy. She knows how to draw a good fight sequence, too, and these tend to be both flashy and clean
(that's something I appreciate greatly, considering the incomprehensible messes I constantly witness with the likes of Naruto, One Piece, etc..).