The Story: It's H.P. Lovecraft meets Monty Python, complete with ancient gods, dark rituals, clowns, hobos, and evil mimes. Two great tastes that go surprisingly well together. Being able to create my own character who features prominently in the cutscenes really made me feel like I was part of the twisted tale.
The Combat: It's as if they took the best bits from several RPG's and tossed them all together, adding their own unique spin. You get standard attacks, super moves that rely on mini-games (one rather similar to Shadow Hearts' wheel mechanic), timed blocking, and support characters that can unleash devastating and not-so-devastating attacks. It's an issue of Popular RPG Mechanics.
The Humor: To be honest here, I am a big fan of the Penny Arcade web comic, and that same style of humor oozes through every pore of PAA. It can be a bit hit or miss at times, but I found myself at least tittering to myself a good 80% of the time, with a few definite LOL moments. "What. The. Fuck."
The Brevity: With RPG's getting longer and longer with each passing year, it's quite refreshing to be able to sit down with a game in the genre and be finished in 8 hours.
The Music: The instrumental music by Jeff Tymoschuk captures the off-kilter atmosphere of the game quite readily, but the ending credits song "Final Boss" by MC Frontalot steals the show.
The Environments: While the various settings in the game are well-detailed and crafted lovingly, there are only four of them, with one of those consisting of a single room. I wanted to see more of the world.
The Item Collection: The game would have benefited greatly from some sort of item store. As it stands, using up all of your power ups during a difficult fight leaves you running back through the stages whacking trash cans in order to rebuild your supply.
Easy Peasy: Enemies are on screen for the most part, allowing you to avoid them while you hunt for items to help do them in, and your health replenishes completely after every battle, making the game a bit easier than I would have liked.
Penny Arcade Adventures: On The Rain-Slick Precipice Of Darkness is a game lovingly crafted for fans of the web comic, so it goes without saying that if you despise Penny Arcade and all they stand for then this is probably not the game for you. If you are a big fan, then by all means, pick the game up. Even if you aren't big on the RPG format, it's packed with tons of trademark PA humor that anyone who follows the comic shouldn't miss out on.
With that in mind, I found On The Rain-Slick Precipice Of Darkness to be a wholly enjoyable experience and an excellent beginning to the Penny Arcade Adventures saga.