In addition to the properties of a rollup, a modular execution is different because it also has these two additional properties:
With a modular execution layer, users can run an off-chain light client, allowing users to validate the chain without running a full node.
A modular execution layer is not tightly coupled to a specific layer 1, allowing for greater flexibility.
One of the core characteristics of a modular execution layer is related to the trust model of the system. A modular execution layer has all the properties of a rollup, and the ability to run a light client. This allows you to have trust-minimized infrastructure. Read more about light clients here.
Another core property of a modular execution layer is that it is not tightly coupled to a specific layer 1. This offers flexibility that allows Fuel to be deployed. Read more about Fuel's flexibility here.