In fact, you can go further: don't try to turn an already-created app into a game. Instead, when you're first working on a business concept, make sure that you create a game first and foremost, and then try to figure out how to let the user do some interesting thing (whatever your original app would otherwise be about) as a side-effect of play. This is how Flickr got off the ground: first it was just an MMO, and then people possessed and traded photos within the MMO, and then that started to become more and more the focus. It's still a game at heart—just like Reddit and, to some extent, Hacker News itself are—it's just shifted to achievement by means of photos, rather than whatever else it had people do before, because that's what interested them.
Let me clarify, though, what I mean by "game." There's this idea in pen-and-paper RPG game design called the threefold model. Basically, it states that there are three reasons people immerse themselves into a virtual, social world constrained by rules (a "game".) It's usually phrased, though, that there are three archetypical players, one to purely seek each goal, of which any individual may be a varied mixture.
1. There are Simulationists, who want to explore the "reality" of the world itself.
2. There are Narrativists, who want to feel that they're part of an overarching story, and that both themselves, and the world, are important and have purpose.
3. There are Gamists, who want to compete for resources, usually against the other players or the game's universe itself.
This model is useful for more than pen-and-paper RPGs, obviously; the three motivations people have for playing games are the same as the three motivations people have for doing anything, once they've sated their base urges: satisfying curiosity and feeling in control of the world (Simulationism), finding a sense of purpose (Narrativism), and feeling powerful and having control of the other players (Gamism). Whenever you're helping people to achieve one or more of those three goals, the means by which you're doing so can probably be classified as a game: D&D, Facebook, the Stock Market, Religion, etc.
A lot of the points this presentation lists (make it a game! reward with points and levels! make users curious by hinting at hidden information!) are techniques that have been used extensively by the most successful Facebook applications. I look at Facebook apps as a bit of a primordial soup of Darwinian evolution, because of the short development cycles and easy access to a potential audience... and the fact that the presentation sums up many of the techniques I noticed succeeding on Facebook is what makes me take it seriously.
Highly recommended for anyone working on consumer apps. Even if you know a lot of the concepts already, it brings them together nicely.
