Why 100 stories?
 

I wonder if anyone out there can answer me this: why is it that for many years all new 'tallest' skyscrapers are just over 100 stories? The Empire State Building was built about 80 years ago and IT is 100 stories.....surely by now they could have been building well over that, except of course for the Dubai tower.

Are there reasons for this?

The short answer is that we like big round numbers. Buildings with bigger round numbers, like 150 or 200, are not really economically viable right now, unfortunately. Burj Dubai, with its 200+ stories, is largely empty and having money problems. The technology is there to build way higher, but the economic justification for it is not.
The Empire State Building has 86 "real" stories. The tip of what was designed as a mooring mast for dirigibles brings the height to 102 stories. That is now just used as a secondary observation deck, which I recommend to anyone who hasn't been up there yet.

Also many floor heights have increased. Today's buildings have a 15 feet ceiling to floor height while 40 years ago it was a 10 feet ceiling to floor height.........

Depends on what the developers are interested in too...

Engineers have been working on designs for 100-200 floor skyscrapers since at least the early 80's. However, they tend to be pretty uneconomic. Currently, the economics tend to fall off above ~80 floors. This is why the tallest towers proposed/u/c in NY and Toronto fall in this range(even 1WTC). Internationally, when a developer wants a landmark tower, and they want to stretch above that limit, they might as well stretch just enough to get the magic 100 number, or in Asia, to 108, 118(Hong Kong ICC), or 128(Shanghai Tower) floors, because 8 is considered a lucky number. This also explains why there are relatively few 90 floor towers.