One of the trickiest problems any ARG designer has is in judging the difficulty of puzzles. Putting aside the question of whether puzzles are actually desirable in ARGs, they provide a degree of interactivity in the story that - ideally - all players can enjoy.
But here's the issue. Many experienced ARG players, especially those in Perplex City, are very good at solving puzzles. They can recognise certain forms of puzzles extremely quickly and have practically internalised the Have you Tried?... list (PDF). They're looking for challenging and original puzzles, and even better, puzzles that require teamwork. However, just catering to experienced players would intimidate newer players, or simply players who aren't as in to the puzzles. So we're always trying to walk a fine line in terms of puzzle difficulty, and it's not easy getting it right.
On top of this is the fact that puzzles are often used as 'blockers' to prevent players from getting to the next juicy bit of content. Sometimes this is done to give the poor ARG designers a bit of a rest (although I think everyone is trying to prepare puzzles in advance these days), and sometimes it's done because it suits the storyline - we want a particular task to take a couple of hours or a couple of days to complete, maybe because there are other events happening that'll take that long as well.
Of course, it doesn't matter what we want - people will solve puzzles in their own time. I remember asking Eric, one of our puzzle designers, for a puzzle where players had to find the Third Power's base of operations. He came up with a rather nice network puzzle, which he confidently said would take at least several hours, and maybe a day or two, to complete. Certainly the players would have to write a computer program to solve it.
Within about two hours, we had an email from someone who'd solved it by hand. Maybe it was luck, but Eric still felt pretty bad about overestimating the solve time. On the other hand, I was very pleased with the puzzle - judging by what people were saying on the forums, a lot of new players had enjoyed the puzzle, immediately understanding the task and seeing that they had a chance at solving it. Sure, it was easier than we thought, but it was also enjoyable for an awful lot of people. That's the ultimate criterion for success.
While planning the single-player parts of Season 2, we spent a lot of time thinking about puzzle difficulty, using our experience with adventure games as a benchmark. I'm not sure whether we've gotten the difficulty correct, and I'm certain that the puzzles will be too hard for some, and too easy for others, but I think we've put into practice a lot of what we've learned from puzzles in Season 1 - plus a few ideas completely novel (and probably scandalous to some) to ARGs.
What are your thoughts about puzzle difficulty? What puzzles have you enjoyed in Perplex City and other games? How are they best tied into the story in a believable manner? We'd love to hear your thoughts.