Ever since I played Brain Age on the DS, I've enjoyed Sudoku. I suppose I'm a logic-puzzle kind of guy, as games like Minesweeper and Picross appeal to me as well. It's a nice way to cap off the evening. I hop into bed, turn on my phone, go through a bit of Sudoku and then its off to dreamland. But I also admire Sudoku for being such a simple example of great design pacing.
Intentional or not, the game has a really nice way of unfolding. The game usually starts fast, with lots of easy-to-find numbers, and it gets slower and slower until it really crawls, and then towards the end it starts to let up until there is a relentless pace of filling in numbers and BAM! it makes you feel like you are the most amazing Sudoku player that ever lived. See how fast you got at the end there? Just one more... Oh yeah, so fast... Now waitaminute... etc.
It essentially has the pace of a good plot, with a good hook, challenges for the protagonist, until there really seems no way out, then something occurs to you and just like rocketing towards a climax, it all resolves. I try to keep this in mind when observing games, too. It's really nice to be lured in, challenged, and have some activity towards the end that just rolls out at a super-fast pace that just makes you want to start all over again. I suppose I'm mostly describing Flow, but still... fun stuff.