Since it's pretty interesting what changes were made on game's maps between 2003 leak and it's November 2004 release, I made these comparisons of official screenshots from various sources and retail version. Currently there're 15 of them in total.
As you can see on early maps the lightning was different in general. Many details and brush structures don't exist in retail anymore, not to mention older citizens models, AR2 and other well known stuff.
And well, this thread can be used to discuss this development era of Half-Life 2 in general.