Links: Nigoro Games
Flash games tend to be hastily programmed, with very little in the way of polish or refinement. This is understandable, I suppose, given the limitations of the medium, but even a simple game can sometimes be made enjoyable by using high production values. Nigoro Games' wildly popular Flash titles (the site is in Japanese, but it's not hard to navigate) are a good example of how to leverage good art and music to enhance a game.
Take "Rose & Camellia", a game that thrusts you into "womanly combat" - slap battles! The concept is strange, but once you start slapping people, the laughs begin.
"Death Village" (think "Lemmings" mixed with a bit of goofy horror) is even more finished - it even has a multi-stage tutorial.