Have you ever felt overwhelmed by the amount of information, the diversity of product offers or the multitude of choices to make? One option is to rely on recommendations of a friend or opinions of other users. Another mechanism is scarcity. Indeed, we attribute a lot of value to things of limited availability or that are being promoted as scarce. It's an old advertising principle: "This category is sold out, 14 remaining of that one". Hurry, or you'll be too late. And who wants to lag behind?
If we transfer this principle to Human computer interaction, it makes sense to limit the options because scarecity may enforce quality! A good example is "Foodspotting", a community site where food lovers can post their favorite dishes they took pictures of. The twist is that you have limited "awards" to hand out and you have to earn them by being an active member of the community. Result? You can assume that the dishes with high ranking on this site are really worth the detour.
Another well-known example of scarcity is the famous 140 character limitation imposed by twitter, (originally due to SMS limitations) which has generated entire seminars around the question on how to best formulate your tweets to reach your target audience.
Imagine if people on Facebook could only give one "I like it" per day...wouldn't that be a lot more meaningful than what we have today?