Hard Activities Discourage Participation
Virgin Media Pioneers, an online community for young entrepreneurs in the UK, launched with one fatal flaw. For users to interact, they had to record videos of themselves and post them to the site.
That's a big ask. First, the participant needs to have the right equipment. Then they need the knowledge to record the video. Then they need the confidence to be on a video. Finally, they need the motivation to keep doing it.
Not surprisingly, it didn't work out well (it's thankfully changed now).
This is an extreme example of a common problem. The more you ask members to do, the less participants you have. Sometimes this can work well, you just get the core group of dedicated members that you're looking for.
Most of the time it kills your participation levels.
I dislike communities that are based around members writing blogs to each other for the same reason. It kills the levels of participation. It's harder to do, less common, and there aren't many successful examples. This is also true of pictures (although some photo-based communities are thriving).
However, on the other end of the spectrum, asking members to do meaningless things doesn't work well either. If you're only asking members to like, vote, pin, retweet, share, and customize, you're not going to get high levels of sustained participation. These activities don't build bonds between members. They're not the type of investments into a community that keep people participating.
Nearly every successful community I can recall asks for members to post shortish messages to each other. You can add in easier and harder activities than posting messages later. However, nearly every community should be based around short, simple, messages.
If you're planning to deviate from this, you're taking a huge risk.