anon (1d AGA): No blind spot for me. For "to kill" problems, the last few (5 or so) moves put the problem in one of a few categories. This problem falls into the snap-back (or double snap-back) variety. Another type is the "shortage of liberties on both sides", whereby the killer splits the killee's group down the middle and the killee can't approach the killer's inside stones from either side. There are other varieties (perhaps a total of 5 or so) and I don't have names for these categories; I'd be hard-pressed to describe them all with text and I don't have the time to make diagrams. But, after doing many L&D probs I have learned to see them. I find searching over a small set of categories easier than a larger set of moves. I can usually guess the category quickly (in fact, I seem to do this subconsciously so I don't really have to ponder "gee, what is the category" -- my mind just jumps to "smells like a snap-back" or "big eye") and that suggests the move. If I can't find the move, I probably have the category wrong.
What makes a problem hard (for me), then, is backing up a few moves so it isn't clear what the category is. Or, there are those problems that really really look like they belong in category A but are really in B so that move that seems like it should work doesn't and I have a blind spot for the one that does. It takes a while to convince myself that I have the category wrong. That I'm putting the problem into a category subconsciously makes it all the harder to get my mind to discard that wrong line of thinking. That's the blind spot effect. I overcome the blindness only when I become conscious that I'm looking at the problem wrong, trying to force it into a "shortage of liberties" type, when it is really a "placement type" or something. When I can't guess the category I have to do a lot more work, maneuvering the problem into one of the categories and then killing.
Velobici: Beutiful problem...rather easy if on has been doing a lot of Life and Death problems. The theme is one that is stressed in Life and Death. Perhaps due to so many people having a blind spot regarding the solution. (Could this be a 9k level indicator?) To solve the problem, one has to break certain rules or heuristics that one develops while playing go at the double digit kyu level.
Bill: As for category, one thing that makes a problem nice is that it does not fall cleanly into a single category.