Well, I was reading in Justin Achilli's blog about this nifty thing called the 90 / 10 rule (which he gave credit to another blog and so on and so forth). Anywho, what it basically means is that the best way to make things weird is to keep it 90% familiar and then twist it 10% of the way. I really like that. It's good because it acknowledges certain tropes that we've come to love and expect and that subverts them to make us sit up and take notice. It's also cool because often when you try to come up with a 100% new idea you either come up with nothing, come up with something hopelessly ridiculous, or just plonk down a trope from another genre that really shouldn't be there.
The 90 / 10 rule can also be used for horror - especially weird horror. If everything is too different, too terrible, too cruel, then it loses a lot of its impact. Often, some of the scariest things are 90% mundane, expected to be there or even beneficial (or appears to be) and then just have that 10% of scary. Oh, look, what a cute little girl playing on the playground ... wait, why is the swing set made of bones?
A pet shop full of toad stools is weirdly creepy and totally unpredictable. Who'd expect toadstools ina pet shop? A pet shop full of pets (predictable) that are sprouting toad stools due to an infectious fungal disease that can spread to you ... well, that's a lot scarier. It's easier to put yourself there and envisage the familiar turned hostile and that clash catches our attention quite nicely.
Anywho, bear it in mind when you're GMing. Heck, Justin's examples include a lot of Fantasy Generic turned into Fantastic, so if you're more of a Fantasy Fan, you'll find some neat examples over at his blog article.