Atleast for me, a game that has legs, is a game that can do one of 2 things:
#1. Have a great opening, then legs atleast 3x or 4x that of the initial launch week. In Japan, many RPGs, despite having great openings (ie, Dragon Quest, Final Fantasy) have abysmal legs.
#2. Have a mediocre start, and then subsequent great sales for a long period of time.
Traditionally, some Nintendo games have great legs - Mario, Zelda, and others are great franchises with strong brand name recognition. So when someone picks up a Nintendo system, any Nintendo system, some games will have great brand name recognition, are of good quality games, and will be purchased late into the buying cycle.
New DS games such as the Brain Training series and Nintendogs are becoming this way, with beastly legs, months and years after the original IP comes out.
However, I would disagree that legs have to do (sometimes) with pricing and/or hardware sales. Generally, the audience the game, or system sells to, control the legs of the games. More "casual" games and lighter-core games tend to attract people that want the game, but aren't willing to pre-order, or go through any hoop to buy the game.
Even some very strange games (ie, M rated), such as Grand Theft Auto games have had brilliant legs - all 3 GTA games on the Playstation 2 had strong sales the first week, but had great legs afterwards. San Adreas opened with 1.5m first month sales in the US, and still sold over 8,000,000 copies, if not more - a multiplier over 5x.
However, hardware and price can come into play as well. For the DS, Wii, and other strong selling systems, when there is an ever-growing userbase with few quality titles, certain good "gems" will invariably make their way through the supply chain, and people will pick them up. Resident Evil 4 is a great example. At a $30 price point, on a fast-selling system, has allowed the game to chart around the #50 position in the NA charts since it's debut. Very, very good.
But legs aren't just Nintendo only, it's for every system provided they have casual and lighter-core titles. Even Rare's Viva Pinata on X360 is seeing a golden age right now in sales - sales this Christmas are almost on-level with last Christmas in the US. The reason? People want the game at $20 as a Platinum title, not a $50 or $60 title....Which is why the game has sold 10,000+ units since November (and failed to chart from April to October, while the price was still sort of high).
Back from the dead, I'm afraid.
