In a few months?
- Simple rules. You want evidence of playtesting and balancing. This is important, and you can't have complex rules that take a year or so to work out.
- No generic system has ever been good, so have a good setting. The setting should compel people to play it.
- The elevator pitch / back-of-the-book blurb should draw people in, and make them think "I bet there are interesting stories I can tell".
- The setting material in the book should make it dead simple for any novice to come up with a basic story /without mechanics/. They might not be deep, but they should be easy. D&D: "Let's explore ancient ruins!" OWoD Changeling: "A chimerical critter is running around being strange, let's cajole it back home before things get too weird for the humans!" Firefly: "Let's have a trading run that inevitably turns into a firefight!" Deadlands: "You're on a train. HORRIBLE THINGS HAPPEN, try to stay alive."
- Decide whether this is primarily a dramatic system (World of Darkness), a heroic system (D&D 2nd), or a wargame (D&D 4th).
-- Dramatic: Combat should be smooth, with relatively little 'crunch'. The outcome of fights is often determined by being smart ahead of time and working things to your advantage, or bringing in NPC allies, or talking your way out. Make the fights short, make sure the characters have social stats, and make sure the game mechanics let them do clever things without being "gimme"s. Make sure the players know that there's a real difference between the suave gentleman trying to talk his way past a guard, and the uncouth brute. You probably want to keep rolls from being too random, though. Being able to rely on your skills is good, and partial successes are nice.
-- Heroic: Combat should be 'crunchy'. The outcome of fights is determined by your tactics. Underpowered PCs can own big opponents if they are smart and use their resources. Give the players options in fights, but don't bog them down...if it takes a couple minutes to work out your options, people will dread getting into fights. Skilled players should be able to take most turns in 20-30 seconds easy. Make sure to allow improvization (what fun is a game where you can't swing from a chandelier?). A bit more randomness is good, because it forces players to react to unexpected circumstances. Pass/fail rolls keep the game moving.
-- Wargame: Don't, they aren't any fun. I'm biased.
- Have an example of play that treats the players as actual human beings. "Fred decides his archer needs some more space, so he runs over to the boulder and takes a shot from there. He rolls X, and Y happens."
- Provide sample places, people, monsters, items. Things that should be common knowledge to the players (so they can skim and get an idea for the setting), and easy for the GM (so they can pick some things and drop in).
- Provide a sample adventure or two, with lots of extra details, fleshed out characters, and lots of extraneous things. It's very, very useful to have some spare personalities lying around; if the players get off track, it can be a lifesaver to have a couple interesting people to talk to. Your average hardcover rulebook will have about ten pages of this in the back...don't neglect it.