It seems as if some of the ancient structures, even more the "wonders" would seem to ambitious for modern construction crews and technology... Are there any that they really couldn't build now, but somehow did then?
There aren't any ancient structures (that I know of) that we really "couldn't" build in their original form in the year 2014 AD. But unskilled labor is -- in most developed countries -- very expensive compared to the cost of machines and materials, and many materials (especially glass and metal) are comparatively much cheaper than was the case in the ancient world. Thus, we today regularly construct very large and complex structures -- like sports stadiums and skyscrapers. But we do it with hydraulic machines and steel, not drystone masonry and vast amounts of unskilled labor.