The Instagram guys ditched the requirement for a little button click and replaced it with a big touch friendly gesture. Two taps and bam, a heart pops up and you’re done. Very satisfying.
Hiding important functionality behind gestures may be something that users evolve to learn, but the road there is still long (I was watching some women exclaiming how hard Instagram was to use the other day!)
Is it worth noting here that although you can use double-tap to like, there’s also a “like” button under every photo? It’s possible to imagine the app without it* but it works well as both feedback that the like worked and as an alternative for anyone who hasn’t discovered the gesture yet.
* Even without the indicator of the button changing from “like” to “liked”, you can tell if you’ve liked something by checking if your name is in the list of people who have. That list collapses for photos that are popular, but it’d be possible to work around that by showing something like “blech and 25 others liked this” for those images.