I've always found Focus Trap (Catch in Focus -- who ever heard of a company changing the name of a feature to one that's longer with more words?
) a Pentax-centric feature that is extremely useful at times. I don't use it that much, but it's very nice to have it in the feature set when it would otherwise take some incredible timing/reaction speed to get a well focused shot of a moving subject.
I don't know if it's Pentax exclusive, but I've also found Multi-Exposure with Ev comp to be a very useful feature.
My most used (by far) Pentax exclusive (other mfgs have made them, but non are as easily used or as universally functional) is the F 1.7x Auto Focusing Adapter. As a long tele user, this has saved me multiple thousands of dollars in lens cost and many pounds of carry weight over the years. The AFA adds focus limiting and increases focus lock speed on any tele lens that's fast enough to utilize it, and for dedicated macros, it increases magnification by almost 2x and limits focus range to only a few mm at MFD, making AF a practical alternative for moving subjects. Of course, the caveat is that it needs to be used with premium glass for the best images and functionality.
This last falls into the same kind of category of Pentax exclusive system "accessories" -- the Q system with an OEM K to Q mount adapter. This stretches both the reach of premium tele lenses and the apparent magnification of macros (my primary pursuits in photography), again without breaking the bank or my back.
Scott