Why since 1997? Because I haven't been in NYC since 1997. I lived there for a few years. It hurts, hurts, hurts to think of seeing the city without the twin towers, which were there every time I was there, beginning in the 1970's when they were still pretty new. Besides being huge they are... They were. It hurts. They were unmistakeable, grabbing the eye from tremendous distances away when the view wasn't obstructed by other buildings -- in many places in Manhattan and downtown Brooklyn the only far-away thing you can see is the sky -- or weather or hills.
But I gather that that loss isn't the only great change in the skyline which would confront me if I went back. On the first page of the 2010 edition of the AIA Guide to New York City, a terrific book by the way, Norval White and Fran Leadon inform us that "September 11 was followed by an unprecendented building boom." I hadn't known that. But I've surfed around and seen some recent photos of Manhattan and, yes, things do look a bit different. On Skysraperpage.com, a nifty site with lots of drawings of the world's tallest buildings, already built, under construction, proposed and more, drawings all to scale next to one another, I refined the search to buildings in New York City, New York, United states, since 1997, and I see that since I've been gone, 3 buildings over 1000 feet tall have gone up in the city (2 according to this website, but I'm counting One World Trade Center as done and they're not counting it as done yet.), 7 over 800 feet, more than 25 over 600 feet, more than 50 over 500 feet. If we expand the search to include buildings under construction we see 6 buildings over 1000 feet, 13 over 800 feet, 37 over 600 feet and Ohmygosh over 500 feet. Yes, things have been happening. The AIA (American Institute of Architects) Guide gives pointed and interested opinions of buildings, but not necessarily always much idea of their size, so the Guide and Skyscraperpage.com sort of compliment each other.