For example, the grass colour changes. Along with everything else. Four years is a long time.
The world file belongs to redditor bmach, who shared it on the r/minecraft subreddit yesterday, showing off what his first map looked like after four years' worth of building and terraforming (using a combination of actual in-game building, and the third-party editing tools MCEdit and Worldpainter).
bmach created the comparison album after accidentally coming across an old collection of Alpha screenshots. To demonstrate the changes, he stood on the exact same spot as in the old shots, taking a screenshot while facing the same direction. You can really tell it's the evolution of the same map on a few of the comparisons:
In others, not so much:
You can find the original, unedited screenshots below. Hunting for references to and stuff left over from the old map is pretty fun.