On a first look, the architectural changes: The picture showing the building which is now the famous Plaza Hotel in New York, with the south-east Central Park corner, presents such a completely different neighborhood than today, that I couldn't recognize it first. Everythings has gotten bigger in New York. Also look at this fantastic picture of the Eiffel Tower with the Trocadero being huge compared to the current one. It's gotten smaller in this part of Paris. Also look at these pictures from England with a falling roof, the same place that would live an housing bubble hundred years later.
You can spot some pretty big cultural changes also. Europeans were clothing very differently, most of that has been lost in between. But the most impressing pictures were the ones from the Iran and Iraq, where you can see how women are clothed in a modern way - if you compare the way women are clothed on these photos in Holland and Iraq, it almost gets incredible. It should remind us that cultural changes are not always getting forward.
Finally, some other photos are not quite so happy. War, destruction and death, that was just the beginning of the 20th century which was quite violent. It's important to have memories of that though, and colour pictures are the best way to remind us that all that was real and not so far away.