The below answer is incorrect. ESX is and always has been a standalone environment. What the original answer refers to is GSX, which is now known as the “VMware Server” product.
I know that there are technological and feature improvements between versions, unfortunately I can’t find a listing of what they are (but perhaps a Google search on release notes might uncover the answers).
I’m fairly sure that the big change between ESX 2.5 and 3.0 is tighter integration with VirtualCenter, and was the first heavy usage of the term “virtual infrastructure”.
You can think of ESX 2.5 as identical to Microsoft’ Virtual Server in that the “virtualization application” is run from a base (pre-installed) operating system. ESX 3.X becomes in essence the “operating system” and “virtualization application” bundled together. Its more fine tuned for performance, high availability, and manageability.