Some of the sources of complexity:
Communication styles & skills
Mandate and support within the organization
Skill-level of the team(s)
A clear goal and/or vision
Technical skill of developers
Number of people involved
Organizational culture
Knowledge-sharing
Process
Accessibility of stake- and shareholders
Involvement of the customer and/or end-users
Tools
Volatility of the market
Time-management skills
Flow
Motivation
Temporal factors (like how long will the project run)
Existing codebase (quality, size, knowledge)
Relationship with suppliers of important components
See “The Three Horizons – Providing a Common Language in its Innovation Use”, Paul Hobcraft, http://bit.ly/1IZeDXN
Examples of companies who use this sort of approach: Intuit, Pearson, Autotrader UK, …