There are great points here already, but for my one essential I'd go for: all children need to experience finding something daunting, working hard at it, and succeeding. Most children can get that with the ordinary curriculum. Often, gifted children can't: they succeed at the normally expected tasks without effort, and the "enrichment" activities they're then given are often fun but not challenging, so they don't help either.
You could tie this in with the previous point about 2SDs above/below by pointing out that children who are working below expectations often have the other version of this problem: they don't get the "and succeeding" part unless someone adapts the task to be appropriate. Gifted children don't get the "finding something hard, working hard at it" part. Both parts are essential.
