How has the basic brick changed over the years?
I can't provide a list of all the minor changes, but I can at least tell the most significant change made in the history of the basic 2x4 brick (which applies to all other bricks and parts, too):


1963: Material changed from Cellulose Acetate to Acrylonitrile Butadiene Styrene (ABS)*


In general, there are no essential changes made: a brand-new brick still fits together with one from before 1963 as long as the Cellulose Acetate hasn't deformed too much during ageing (which is the main reason they switched to ABS, it's more robust, scratch-proof and doesn't deform so easily).

The main "change" that really affects building is the continuously improving precision in production - this happens all the time, so there's no point on the timeline. The effect is simple: better fitting bricks improve the stability of your construction.

*Transparent bricks consist of Polycarbonate, axles consist of Polyamide