Now, I gave Shakugan no shana a 4/5. This is manly due to its engaging storyline and interesting characters, something a action/slice-of-life need to have. That being said, that 4/5 rating only applies to the English dub, not the Japanese dub.
Now you may be thinking, but isn't the English dub always inferior to the original audio. To that I'd say, what "original audio"? you mean the voices of the computers used to animate this anime? Then you'd probably slap me and tell me you meant the Japanese dub (or probably the sub, but then I'd have more tangential things to say) but then Id say that after enjoying this anime, I tried it again with the Japanese dub since I'd have to watch it that way for the second season, and I noticed something strange.
I had suddenly started hating all of the characters. It seemed that the differences between the two dubs, though subtle, change a lot for the better.
Let me break it down like this. The characters, namely Yuuji and Shana. I don't know why I didn't notice before, but Yuuji's entire motivation is existing. Sure, it does kinda work for a while but he very quickly turns into a Gary sue whose sole character trait is his love of existing. So when he's placed next to better characters like shana, his paper lines start to show, But at least the English dub's actor somehow found enough motivation from somewhere to make it should like yuuji had something else to do all the stuff he does for otherwise no reason.
Now for shana, I could just type the words Rie kugimiya about fifty-one times and leave it at that, but I'd consider that a bit too much joke and not enough actual explanation. Shana's Japanese voice actor is one of the most annoying higher profile voice actors out there, listening to her is like shoving pure molasses into your ears, it sounds sweet at first but it gets annoying fast. Now I'm confused onto why the casting people thought a flaming badass action girl should have the vocal equivalent of melted sugar, I mean, she is small and tsundere, but that doesn't mean you should cast the same person you always get to do small and tsundere for it. Because of this, it really doesn't fit when she starts slicing things in half and setting them on fire at the same time. The English VA does much better, she actually sounds like a girl who plans on doing nothing but beating up bad guys, so it becomes appropriately cute when she doesn't (unlike the Japanese dub, where you were expecting it from the start)
Now for the rest of the cast, I never really liked Marjory and thought that her voice in both was forgettable. But her book, marchosias. Oh my god, Marcho, I loved this guy, he had this sarcastic tone that seemed to always have this slight hint of "If I weren't a book I'd effing kill you all!" But when I switched the the Japanese dub, I noticed that wonderful hint of madness was gone, leaving him as a dull sarcastic book that likes to say the phrase "My XXX marjory daw" And Kasumi Yoshida, would I say the English dub made her likable, no, she's beyond help, there is not a single plot point she opens up that is interesting in any way. geez.
So Yeah, the English dub gets a 4/5 the japanese get 2.5/5. It loses the 1.5 because a slice of life anime needs good characters to survive, so it would lose half of its points for losing half of its plot. Fortunately, this isn't the SoL heavy second season, so it doesn't lose too much.