The Seven Deadly Sins has been a meme since long before there were memes.
Since they were canonized by Dante, seemingly every philosopher/writer/poet has at some point featured them in a essay, story or poem. This is because they so succinctly encapsulate the motives of every terrible thing ever done by a human being to another.
More than that, they describe the motives of everything ever done, good or bad, because what underlays them are the raw fundamental forces of our psyche. You could describe personal fulfillment as somehow finding a homeostasis between them, lest one runs rampant and turns deadly.
So what can a mere product learn from all of that? Here's a playbook for channeling each into lasting engagement.
Okay, this one is a layup. Everyone is lazy. The busier you are, the lazier you must be, because every moment wasted on a clumsy tool is magnified 10 or 100-fold.
Let your users be lazy.
Go through every important flow through your application. Sculpt and shape every action to minimize time and reduce the number of decisions. Strip away anything and everything until you can no longer remove any of the parts. Help your users to magnify and amplify the results of every action they take.
Users always want more more more. Whether, features or content, they complain endlessly about not having the button that does that thing, or why the doodad they were looking for wasn't conveniently on the nav bar. Usually you fend them off, with a stick, because they can't see how it would harm the core value you provide, but once in a while, they are right.
There are elegant ways of giving it to them.
Any content driven application without great search should be ashamed of itself. It's one of the harder things to pull off because some content requires a lot of curation to make it searchable (video, audio, images) and even text content, especially structured data, has to be finely tuned for the search index to surface relevant results. Spend the energy and time. Use analytics to study how people use it. Find out what they query for and tune your indexes to give better more accurate answers.
Feature laden applications, particularly those with necessarily dense drop down menus and preference pages, should focus on plugability and extensibility. Is there a good reason why you need to build a calendar into your application? Why not integrate with a Google calendar instead, even better, support the iCal standard so each user can bring their own. There are a multitude of other products out there that offer richer functionality than you can provide and cost you very little to implement.
Money, truly, is a double edged sword. You want it from them, they most definitely don't want to give it to you. You can give it away for free, but then you debase the currency of value you've worked so hard to create.
Advertising is a way of selling a bit of your soul (and your users souls) to get some of it, and for some products, with large user-bases and fickle engagement, it's the only game in town.
Times are a changing though. It's never been easier to take a payment online and people are getting used to opening their wallets online.
The worst thing you can do is confront the user with a "Buy" button on the first page view. Freemium and trial-periods are useful for widening the engagement funnel, though can be technically difficult to pull off. Largish up-front fees can be split across a monthly subscription to lower the perceived cost and also give you incentive to create more lasting engagement. Pay-per service applications can use first-time free incentives and punchcard-like loyalty programs to gain users and keep them coming.
Most importantly, build value and price appropriately. Value stems from saving people time, facilitating transactions, providing goods/services, or just plain amusing them. Figure out what value you're providing and find a parallel product to anchor your pricing against, online or sometimes physical. Be careful not to mistake having poured your guts out to create a thing for a dispassionate user's willingness to pay. If all else fails, don't be afraid to start a tad high though and use promotional discounts to test lower price schemes.
The sexual imperative is so deeply ingrained in all of us that it subjectively shapes everything. I believe, if Google's search index ever becomes self-aware, it's first conscious act will be to laugh raucously at the human male's obsession with breasts. Why should they be spend so much energy admiring two bodily protrusions that nearly every woman possesses, that are only really useful to newborn babies and otherwise cause women a great lot of trouble supporting, covering, revealing, augmenting, reducing, and sometimes risking their lives for.
And yet, if you're a man, then nothing in creation could make more sense then that elegant equation that describes the gentle curve from shoulder to stomach.
This does not mean you should put tits on your homepage.
Rather the opposite. Sex is too powerful a force for you to wield responsibly. Respect it. Respect that a hormone addled teenager will inevitably post pictures of their penis on your site, respect that some people will want to have open discourse about subjects that others (or even you) will find offensive, respect that some people will want to pick the username "BallsDeep", respect that no viewpoint is invalid and foster a community of mutual respect. Most especially, respect their privacy.
There must be a thermodynamic equation for hatred. I imagine that a college student "hates" having to wait 30 minutes for a pizza about exactly as much as a cavemen hated having to walk 5 miles to hunt an antelope. This isn't to say we're spoiled, (though we are), more that we all have a certain amount of cussedness that has to be let out one way or another.
As a product owner, there are ways of channeling and harnessing that energy to create healthy engagement. Providing a soap-box for your users to vent, whether about the driver who cut them off on their way to work or your product, is a way of delivering value in the form of emotional release.
When their ire is directed at your product, it's an opportunity to learn from their pain. Beware becoming a sycophant that react to every barb, but there is a sort of judo where you can turn your biggest detractors into your biggest proponents by simply promising a solution, then delivering.
Envy gets a bad rap for being the most tasteless of all the sins. Jealousy over what you don't have when you've probably got quite a lot, is after all, pretty base. But the desire to compete is the fundamental force of all positive evolution, whether biological, social or technical.
Products must themselves compete for the over-saturated and multi-fractured attentions of users. These days, a consumer-facing web application must compete, not just against similar products, but against every search result, every advertisement and every bookmark.
To do it, products should let their users wear the value they get from a product on their sleeve. Nothing so contrite as a +1/Like button, or god forbid, a forced twitter post. But with public forums and rich public user profiles that showcase their rich experience with your product. When new or potential users see established users engaged with your product and enjoying themselves, they'll want in on the action.
Of all the sins, this is the one I'm most guilty of. The need to feel just a little bit better than the other guy is hard to censor and easiest to luxuriate in. I'm able to suppress it, for a time, in order to learn from and collaborate with peers, but there's still an insatiable hunger in me to do better/faster/cheaper than the rest.
Thus, it is one of the strongest ways of creating lasting engagement. Users who take pride in using your product will form the nucleus for all your success. They will be the standard bearers that spark viral growth on Twitter and Facebook, they will become the stable source of revenue that frees you to continue to take risks, they will brag about how they were there when and wear t-shirts with your logo on them.
People become proud of a product by feeling they've earned it. This could be as simple as a badge for participating in a closed beta, as concise as a counter of posts, or as genuine as having their good idea get implemented and their name credited in the company blog. It's about rewarding your users for their time and energy in making your product successful.