The Ti West flaw
I have only watched House of the Devil and the Innkeepers, but the thing that bothers me most also seems to annoy many other people.
First, here is what Ti West gets right:
1. Reliance on story over effects.
2. Human agency seems to have some ability to affect events. If you stab a human, they stay dead, and don't have a long post stabbing period of superhuman strength.
3. Long shots as opposed to all quick edits and jump scenes.
4. Seemingly intelligent protagonists (until they commit the fatal Ti West flaw)
The Ti West Flaw:
When in danger, the protagonist runs to a tactically inferior place that is also cultural and psychologically the most fearful, countering human behavior. In other words, instead of Getting Out of the Freakin' House, where there are multiple avenues of escape and the opportunity to avoid architecturally based horrors, the protagonists retreat to attics and cellars, where we all know evil things are strongest. Once the heroine does this, I lose all sympathy for her.