What about a game grabs your attention enough to give it a second or even third look? What is it for you that will turn a P.I.G into a purchase?
The first and most important thing for me when choosing a game is to be able to flip[ through it and review what is on pages. This is one of the main reasons I like One Bookshelf's “PayWhat You Want” feature and believe more developers should utilize it. As our hobby moves more and more away from the traditional brick and mortar game stores and from physical books in general the normal ways of discovering a game system go away as well. Typically with pdf and epub the reader is unable to flip through the digital pages and skim to see if he enjoys what is on the page and the risk exists for people to be burned by cool covers and blurbs that don't pan out in the actual product.
This is why the “Pay What You Want” feature is excellent as it allows me to browse the product and decide partially if it will move from the P.I.G. pile to the I.G. pile. By browsing the product I can see if the flavor matches what I am personally into, if the rules are or are not overly complex and if the product as a whole is composed and presented in such a way that “flow” exists in a nice and linear fashion.
First lets talk about flavor. Believe it or not flavor isn't that important in the long run as any rule system can be shoehorned into any setting, genre or flavor of either of the previous. However, when first picking up a flavored system (see Numenera, Eberron, Vampire) the flavor matters because the flavor is part of the reason that the product was picked up in the first place. Yes, I can d20 Modern to play a well rounded Supers game with little modification or I can go and play a flavored version like d6 Powers or Champions. All three can work in the Supers setting, two were created for the pure reason of a Supers game. However, (and note I have not yet read d6 Powers) while d6 Powers is built on one of the founding fathers of game systems the flavor and flavor related mechanics may drive me away … for now … and right into the arms of Champions, d20 or some other system.
In the end mechanics trump flavor for the reason mentioned above. My personal belief is that any game worth playing should be readily playable by anyone within five or ten minutes of the core book being placed in that person's hand. After all, these are games that we're playing and no game should ever feel like homework. This is another reason being able to browse a product is a wonderful idea, because if a potential player is able to grasp the basic mechanics of character creation from a quick flip through then the game is probably worth getting. The simplicity of the rules brings up another point of the mechanics that will change my opinion toward a potential product; rules over story.
Back when I first saw “Gamers: Dorkness Rising” I was utterly thrilled that one of the main poles of the movie was the struggle between the players wanting to adhere to “the rules as written” and the Dungeon Master just wanting to tell a great story. Both sides of the coin where shown, the DM getting railroad-y, the players ruining excellent roleplaying moments with rules lawyer. It was fun to see this battle brought to life outside of the tables of real gamers and brings about my point. The rules to any system should never be so comprehensive that they ruin the ability for a story to be told. The Game Master should be trusted to adjudicate fairly and balance player freedom to his story. Once a game publisher starts to distrust their GMs a lot of potential fun is lost.
Finally the flow of any given product in our hobby matters. I personally tend to look at everything from a character creation point of view. A core book should flow from one step to the next of character creation and in such away that a first time player can fill out his character sheet with out constantly flipping back and forth between the pages. If the core book has a game master, how-to-play, and monster sections then those two should be placed in such a way that the player (the person with a character) comes first. While not the biggest gripe on my end I have seen more than one new player walk away from a table because they couldn't find a straight forward question to how CMD worked.
While flavor, mechanics and flow are important there are other things I look for when trying to discover if a game is worth my time and cash. Chief among these is the thought of how much stuff I need or do not need to play the game. Before the advent of the e-reader and .pdf as the near default means in which the gaming community consumed the hobby the thought of lugging a players manual, a Dungeon Master's Guide and a Monster Manual as well as dice and possibly miniatures would cause my arm and satchel to whimper. Near the end of 4e the books weren't an issue but my DM required fortune cards at his game, miniatures, the expanded character sheets that required an insider account and … too much stuff!
Personally I believe that one book and a set of dice (and maybe a mini) are all that is needed. This goes so far as when I see my brother with his ten pound bag of d10s at a Vampire game I get sad. Seriously, this should play a role in the design process of a game, to make the game simple on every level. A few dice, a reference sheet and maybe a book is all that is really needed.
Hopefully I answered the question of what I look for in a potential game and more-so I hope that my own endeavors into game design will eventually invoke these thoughts and belief.