If I got into the mind of my character, DiTerlizzi's monsters are not things that I want to fight. He retains the whimsical strangeness of a monster without losing the sense of danger. For example, this observer could be read in a number of different ways
It looks like a deep sea creature, weird and alien, but not necessarily hostile. A more
typical depiction would be almost cartoonishly aggressive, with exposed teeth and an
aggressive stance. But this illustration really seems like a random encounter, a creature minding its (alien, incomprehensible) business on its own plane when out of nowhere comes a plane shifting human. He manages to use the single, gigantic eye to create an expression that is in turns curious and predatory, as if the creature is still deciding how to respond. Yet it's also somehow cute and fantastical?
Not that he can't do hostile, as he often does with various lower planar creatures.
But again with these images, I'm not thinking, "ok let's fight," as I might with a monster in a more traditional
action pose. Instead the viewer is more inclined to think, "let's back away slowly...maybe it didn't notice us...now run!"
His NPCs are similarly worth engaging. They are confident, savvy, powerful, and clearly have a lot going on that is not related at all to the PCs. If you were going to sit down with these characters to have a drink, you would have to be both sociable and guarded as you try to figure out what schemes they are up to.
All together, it worked really well for PS's urban portal fantasy, one that was supposedly a lot more about negotiation and problem solving against forces infinitely more powerful than you. (In fact, I'm not sure the setting materials and adventures ever really lived up to that premise, and maybe dnd is the wrong system, but that's another thread.)