A plan is an example of what could happen, not a prediction of what will happen.
The community isn't nearly as afraid as it thinks it is.
Optimism is an occupational hazard of programming; feedback is the treatment.
Sometimes the problem has to mature before the solution can mature.
The business changes. The technology changes. The team changes. The team members change. The problem isn't change, per se, because change is going to happen; the problem, rather, is the inability to cope with change when it comes.
I tell people to start implementing when they are pretty sure there aren't more important stories out there. An iteration's worth of data is worth months of speculation.