This seems to be a difference between what I'll call the Williams tradition and the Jensen tradition.
(note: the only Roberta Williams' games I've played are the KQs ... I don't know what Laura Bow, Phantasmagoria, etc. are like)
Between KQ4 and ... say Gabriel Knight 3 ... TSL feels a lot more like GK3 (and I am not talking about the graphics). While KQ4 has a great sense of urgency, it's like a fairy tale in that one event after another happens, but the events are only loosely tied together (for example, the minstrel is not particularly connected to anything else, he just happens to be there for Rosella's convenience, rather than, say, being Lolotte's spy or anything), and there is no particularly significant revelations. Likewise, the characters are quite simple - we don't get a lot of psychological depth. Whereas GK3 is full of connecting various bits of history and lore together in mind-shattering ways and revealing deeply-buried secrets. We also get a much more nuanced notion of what's going on in the characters' heads.
Granted, KQ3 does have the big revelation about Gwydion's past, and KQ5 does have the plot-connectivity points previously mentioned, but not nearly to the same degree as the GK games ... or TSL.
So I'm basically agreeing with BT that TSL is a KQ6 fan's dream ... since anyone who prefers KQ6 over the other KQs is likely to be more of a Jensen fan than a Williams fan. Note that it was Jensen who dropped the Black Cloak Society bit into KQ6.
I myself prefer the Jensen tradition, so TSL is pretty much my dream KQ fan game. I also like the simplicity of the Williams style, and if this were an official KQ9 game I would want it to be more balanced (a la KQ6), but while playing that official KQ9 I would probably constructing a TSL-esque story in the back of my head.