I read a quote from some author once, wish I could remember who or the exact wording, along the lines that an author "can only tell one lie per work of fiction" (well good fiction at least), e.g. for proper suspension of disbelief, the audience will swallow one fantastical premise no matter how unrealistic, as long as the rest of everything else flows naturally from that premise and common sense. I think this line of thought may be relevant to what Neonevik is talking about, too many unrelated fantastical elements really challenge suspension of disbelief, and start to feel like everything in the show is explained by Deus Ex Machina. It's fine for comedic works to deliberately pile silly ideas on top of each other (which effectively is parodying/subverting well-written works, going against good rules of writing becomes part of the humor), but not for any remotely "serious" work.
Death Note is a good example of a work of fiction that follows this rule: the basic premise of the Death Note's is very simple and easy to comprehend, and the show explores the logical outcomes from that in a pretty sensible way. Occam's Razor might also apply as a concept, less is more - Death Gods write your name in a notebook, no more explanation needed. Sometimes you're better off not bothering with explaining at all, e.g. Death Note vs Code Geass, I still have no idea where and why the Geass came from.