The plot itself is relatively simple. A nearby neutron-star binary collapses, violating the currently understood physics and generating a burst of gamma radiation that threatens the Earth. Two citizens from Konishi polis activate abandoned gleisner robots to try and warn the fleshers of what is to come. Carter-Zimmerman polis subsequently dispatches copies of itself to other stars, seeking an explanation of what happened and some kind of certainty about the future. The protagonists end up chasing a species called the Transmuters (who have left behind them clues such a planet composed solely of deuterium and C-13 and other heavy isotopes).
There is plenty of "hard" science fiction around, but Diaspora makes most of it feel like talc. An explanation of the Gauss-Bonnet theorem and an introduction to gravitational astronomy are just warmups — not only does Egan speculate about 6-dimensional physics and visualisation in 5-space, but he makes a serious attempt to explain the mathematical concepts of "manifold", "embedding", and "fibre bundle"! Some of the physics is invented, but the basic ideas closely reflect real modern theoretical physics. You don't have to be able to follow all of this to appreciate Diaspora, but those without any interest in physics and mathematics are likely to find it rather tedious. (A twelve page glossary and two pages of references are included.)
As a novel Diaspora is limited — though the characters are all recognisably human, none of them ever really comes to life. But the speculations on philosophy of mind are more interesting in many ways than the physics. Polis citizens are created either by copying of a biological brain (using nanoware) or by simulated neuro-embryology. They can directly modify their own motivations (with "outlooks") and perceptual and cognitive abilities, as well as cloning themselves (and, under some circumstances, clones can subsequently merge). Unlike the physics and mathematics, which are necessarily presented in long passages of exposition, the ideas about mind and consciousness emerge more naturally from the story.