Thanks for all the insightful responses; a sure way to warm a cold Saturday in Manhattan.
Tim: Yale's 18th is certainly in a category unto itself, but it also fits well into the type of three-shot par-5 hole I like--i.e., one where the second shot provides options (more than most, to be sure) and demands your full attention and thinking. Bethpage's 4th, as you note, does the same, but with fewer options.
Jason: Precisely, although I have to admit that when I was making that point in my post, I did consider perhaps the lone merit of the boring layup--it's so seemingly straightforward, just advancing the ball down the fairway to set up the third, that it engenders laziness and makes it among the harder shots to execute.
Tom: Any Ithaca-based story brings a smile to my face (my father also went to Cornell), but yours takes the cake. Thanks for sharing. I share your and Tim's preference for a nice variety of par-5s, and I like the words you chose to describe what I called "category 2"--i.e., a genuine three-shotter where the second-shot "assignment is a bit more complicated than it sounds." Many things can add complication and interest to the second shot, which is why I imagine (or at least hope) that most architects find it among the more demanding shots to design for. I think that is essentially Paul's question.
Carl: That is quite the thread; thanks for linking to it. I'll explore it later today and chime back in with further comments.
Peter: Thank you for such a delightful, informative, personal description of the 8th at Crystal Downs. Even before I read your follow-up post, "engagement" jumped out at me as the principle to glean from your first. That's a great way to describe all great golf holes, not just par-5s. Like complication and interest (see my response to Tom above), engagement can come in many forms, but as long as it exists (i.e., as long as the architect doesn't get lazy, like a golfer might, on the second shot on a par-5), the game provides a deep sense of satisfaction. With that said, the principle you threw out in your follow-up post caught me by surprise, but it's a fascinating one to consider. I do often wonder whether a short par-5 (with a long second shot) "must" have a bland green complex while a long par-5 (with a short third shot) "must" have an intriguing one. Your principle gets at why formulas needn't/shouldn't be so formulaic. (And just as I was about to post this response, I see Bill's post, which further proves this point. Ridgewood is high on my list of courses to play.)