Seth Godin writes:
The first is fear.
The fear that you'll have to implement whatever you dream up.
The fear that you will fail.
The fear that you will do something stupid and be ridiculed by your peers for decades.
The fear that you'll get fired.
The fear that there will be an unanticipated backlash associated with your idea.
The fear of change.
The fear of missing out on the thing you won't be able to do if you do this.
The second is a lack of imagination.