A compelling vision is a simple statement, in terms that all stakeholders can understand, of how the migration will result in a substantially improved site. This vision must be concrete enough to prioritize tasks / functionality / content during the migration, and also motivating enough to mobilize everyone (and tolerate the inevitable hiccups).

Without a compelling vision, your migration can get caught in a quagmire or indecision or you may end up with a muddled (and potentially overcomplex) implementation.

Please see Compelling Vision for a Large CMS Migration for more.